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Full text of "NBS solar collector durability/reliability test program : final report"

Reference 

NBS 
PUBLICATIONS 

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NAT'L INST. OF STAND & TECH 



A111DS T7M327 



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NBS TECHNICAL NOTE 



1196 



U.S. DEPARTMENT OF COMMERCE/National Bureau of Standards 



NBS Solar Collector 

Durability/Reliability 

Test Program: 

Final Report 



T 

m he National Bureau of Standards 1 was established by an act of Congress on March 3, 1901. The 
m Bureau's overall goal is to strengthen and advance the nation's science and technology and facilitate 
their effective application for public benefit. To this end, the Bureau conducts research and provides: (1) a 
basis for the nation's physical measurement system, (2) scientific and technological services for industry and 
government, (3) a technical basis for equity in trade, and (4) technical services to promote public safety. 
The Bureau's technical work is performed by the National Measurement Laboratory, the National 
Engineering Laboratory, the Institute for Computer Sciences and Technology, and the Center for Materials 
Science. 



The National Measurement Laboratory 



Provides the national system of physical and chemical measurement; 
coordinates the system with measurement systems of other nations and 
furnishes essential services leading to accurate and uniform physical and 
chemical measurement throughout the Nation's scientific community, in- 
dustry, and commerce^ provides advisory and research services to- other 
Government agencies; conducts physical and chemical research; develops, 
produces, and distributes Standard Reference "Materials; and provides 
calibration services. The Laboratory consists of the following centers: 



• Basic Standards 2 

• Radiation Research 

• Chemical Physics 

• Analytical Chemistry 



The National Engineering Laboratory 



Provides technology and technical services to the public and private sectors to 
address national needs and to solve national problems; conducts research in 
engineering and applied science in support of these efforts; builds and main- 
tains competence in the necessary disciplines required to carry out this 
research and technical service; develops engineering data and measurement 
capabilities; provides engineering measurement traceability services; develops 
test methods and proposes engineering standards and code changes; develops 
and proposes new engineering practices; and develops and improves 
mechanisms to transfer results of its research to the ultimate user. The 
Laboratory consists of the following centers: 



Applied Mathematics 
Electronics and Electrical 
Engineering 2 

Manufacturing Engineering 
Building Technology 
Fire Research 
Chemical Engineering 2 



The Institute for Computer Sciences and Technology 



Conducts research and provides scientific and technical services to aid 
Federal agencies in the selection, acquisition, application, and use of com- 
puter technology to improve effectiveness and economy in Government 
operations in accordance with Public Law 89-306 (40 U.S. C. 759), relevant 
Executive Orders, and other directives; carries out this mission by managing 
the Federal Information Processing Standards Program, developing Federal 
ADP standards guidelines, and managing Federal participation in ADP 
voluntary standardization activities; provides scientific and technological ad- 
visory services and assistance to Federal agencies; and provides the technical 
foundation for computer-related policies of the Federal Government. The In- 
stitute consists of the following centers: 



• Programming Science and 
Technology 

• Computer Systems 
Engineering 



The Center for Materials Science 



Conducts research and provides measurements, data, standards, reference 
materials, quantitative understanding and other technical information funda- 
mental to the processing, structure, properties and performance of materials; 
addresses the scientific basis for new advanced materials technologies; plans 
research around cross-country scientific themes such as nondestructive 
evaluation and phase diagram development; oversees Bureau-wide technical 
programs in nuclear reactor radiation research and nondestructive evalua- 
tion; and broadly disseminates generic technical information resulting from 
its programs. The Center consists of the following Divisions: 



Inorganic Materials 

Fracture and Deformation 3 

Polymers 

Metallurgy 

Reactor Radiation 



'Headquarters and Laboratories at Gaithersburg, MD, unless otherwise noted; mailing address 
Gaithersburg, MD 20899. 

2 Some divisions within the center are located at Boulder, CO 80303. 
3 Located at Boulder, CO, with some elements at Gaithersburg, MD. 



KATlOKAt BUREAU 

OP fftAl*rt»BP» 

UBRABt 



NBS Solar Collector a ^° 

Durability/Reliability Test Program: '%yj ( ^t 
Final Report /??c/ 



David Waksman 

Center for Building Technology 
National Engineering Laboratory 
National Bureau of Standards 
Gaithersburg, MD 20899 

William C. Thomas 

Department of Mechanical Engineering 

Virginia Polytechnic Institute and State University 

Blacksburg, VA 24061 

Elmer R. Streed 1 

Center for Building Technology 
National Engineering Laboratory 
National Bureau of Standards 
Gaithersburg, MD 20899 

1 Now retired. 



Prepared for: 

Office of Solar Heat Technologies 
U.S. Department of Energy 
Washington, DC 20585 

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00 

5753 %*, 



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0, 1196 M 5 5 J ^cAskJratA /1/IC&- 



go^l . DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary 

IONAL BUREAU OF STANDARDS, Ernest Ambler, Director 

Issued September 1984 



National Bureau of Standards Technical Note 1196 
Natl. Bur. Stand. (U.S.), Tech. Note 1196, 148 pages (Sept. 1984) 

CODEN: NBTNAE 



U.S. GOVERNMENT PRINTING OFFICE 
WASHINGTON: 1984 



For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, DC 20402 



ABSTRACT 

Efforts in the development of reliability/durability tests for solar collectors and their materials 
have been hampered by the lack of real time and accelerated degradation data that can be correlated 
with in use conditions. The focus of this report is on research undertaken at the National Bureau 
of Standards (NBS) to help generate the data required to develop methods for predicting the long term 
durability and reliability of flat-plate solar collectors and their materials. 

In this research, eight different types of flat-plate solar collectors were exposed outdoors at four 
sites located in different climatic regions. Small scale cover and absorber materials coupon 
specimens consisting of samples taken from a collector of each of the eight types used and a number 
of additional materials were exposed concurrently with the full-size collectors. Periodic measure- 
ments were made of collector and materials performance as a function of outdoor exposure time. In- 
door laboratory aging tests were conducted concurrently on specimens of the same materials to provide 
a basis for comparison with the outdoor exposure tests. 

This report presents the results obtained in this test program. Recommendations are made regarding 
the use and limitations of performance measurements and environmental exposure tests for assessing 
the durability of solar collectors and absorber and cover materials. 

Key words: absorber materials; accelerated aging; cover materials; durability; environmental exposure; 
solar collectors; solar materials; stagnation testing; thermal performance. 



iii 



ACKNOWLEDGEMENTS 

The work discussed in this publication was funded by the Office of Solar Heat Technologies 
of the United States Department of Energy. Outdoor thermal performance testing and exposure 
of solar collectors and materials were carried out by DSET Laboratories (DSET), the 
Florida Solar Energy Center (FSEC), the Lockheed Research Laboratory (LMSC), and the 
National Bureau of Standards (NBS). Indoor solar simulator tests were performed at the 
Kent Space Center of the Boeing Company and by Wyle Laboratories. The cover and absorber 
optical property data reported were measured by DSET, LMSC, and NBS. Assistance in data 
analysis and evaluation was provided by the Department of Mechanical Engineering of 
the Virginia Polytechnic Institute and State University (VPI&SU). Optical and scanning 
electron microscope studies and PMMA molecular weight measurements were performed at 
NBS. 

Personnel from the above mentioned organizations who participated in the test program 
are listed below. Apologies are expressed by the authors for any omissions. 

Test Site Managers 

DSET: Richard D. Whitaker 

FSEC: James C. Huggins and Ross McCluny 

LMSC: Roger K. Wedel 

Boeing: Al R. Lunde 

Wyle: Wallace R. Youngblood 

Collector Exposure and Thermal Performance Testing 

DSET: William J. Putnam 

FSEC: James C. Huggins and James D. Rowland 

LMSC: Ronald Dammann 

NBS: John R. Jenkins and Donn F. Ebberts (of NBS), 

Ken Shih and Mark Rallas (of Wyle) 
Boeing: Al R. Lunde 
Wyle: Wallace W. Youngblood 

Outdoor Exposure of Materials 

DSET: Thomas E. Anderson 

FSEC: James C. Huggins and James D. Rowland 

LMSC: Ronald Dammann 

NBS: James F. Seiler 

Cover and Absorber Optical Property Measurements 

DSET: Thomas E. Anderson 

LMSC: Roger K. Wedel 

NBS: Willard E. Roberts and Catharine D. Kelly 

Data Analysis and Evaluation 

VPI & SU: Aaron G. Dawson, III, Donald S. Culkin, 
Daniel E. Douro, and Mark A. Monroe 

Optical and Scanning Electron Microscopy 

NBS: W. Eric Byrd 

PMMA Molecular Weight Measurements 

NBS: Brian Dickens and Sharyl J. Santema 



IV 



SI CONVERSION UNITS 

The metric SI system of units is used throughout this report-. This table is included to assist in 
converting from SI metric units to common U.S. units which are presently used by the building industry 
in the United States. 

LENGTH 

1 in = 0.0254 meter (exactly) 

1 ft = 0.3048 meter (exactly) 
AREA 

1 in 2 = 6.45 x 10" 4 meter 2 

1 ft 2 = 0.09290 meter 2 
VOLUME 

1 in 3 = 1.639 x 10~ 5 meter 3 

1 gal (U.S. liquid) = 3.75 x 10 -3 meter 3 
MASS 

1 ounce-mass (avoirdupois) = 2.835 x 10 -2 kilogram 

1 pound-mass (avoirdupois) = 0.4536 kilogram 
PRESSURE OR STRESS (Force/Area) 

1 inch of mercury (60°F) = 3.377 x 10 3 pascal 

1 pound-force/inch 2 (psi) = 6.895 x 10 3 pascal 

1 pound-force/foot 2 (psf) = 47.88 pascal 
ENERGY 

1 foot-pound-force (ft-lbf) = 1.356 joule 

1 Btu (International Table) = 1.055 x 10 3 joule 
POWER 

1 Watt = 1 x 10' erg/second 

1 Btu/hr = 0.2931 Watt 
TEMPERATURE 

t°C = 5/9 (t°F - 32) 
HEAT 

1 Btu/in/hr«ft 2 «°F = 1.442 x 10 _1 W/m«K (thermal conductivity) 
1 Btu/lbm«°F = 4.187 x 10 3 J/kg«K (specific heat) 
1 langley = 4.184 x 10 4 J/m 2 = 1 cal/cm 2 = 3.69 Btu/ft 2 



TABLE OF CONTENTS 

Page 

ABSTRACT t±± ± iii 

ACKNOWLEDGMENTS iv 

CONVERSION UNITS v 

1 . INTRODUCTION 1 

2. TEST PROGRAM OVERVIEW 2 

2.1 Solar Collector Tests 2 

2.1.1 Outdoor Exposure Conditions 2 

2.1.2 Collector Description 4 

2.1.3 Measurements and Observations 4 

2.2 Collector Materials Tests 6 

2.2.1 Exposure Conditions 6 

2.2.2 Test Specimens 6 

2.2.3 Measurements and Observations 6 

3. RESULTS AND DISCUSSION: MATERIALS EXPOSURE STUDIES 15 

3.1 Cover Materials Testing 15 

3.1.1 Full-Size Collector Stagnation Testing 15 

3.1.2 "Real Time" Outdoor Mini-Box Testing 18 

3.1.3 Accelerated Outdoor Testing 27 

3.1.4 Temperature Testing 30 

3.1.5 Temperature and Humidity Testing 30 

3.1.6 Temperature and Xenon Arc Radiation Testing 34 

3.1.7 Comparison and Assessment of Test Procedures 39 

3.2 Absorber Materials Testing 42 

3.2.1 Full-Size Collector Stagnation Testing 42 

3.2.2 "Real Time" Coupon Specimen Testing 42 

3.2.3 Accelerated Outdoor Testing 48 

3.2.4 Temperature Testing 48 

3.2.5 Temperature and Humidity Testing 48 

3.2.6 Temperature and Xenon Arc Radiation Testing 53 

3.2.7 Thermal Cycling 53 

3.2.8 Comparison and Assessment of Test Procedures 53 

3.3 Additional Collector-Level Observations 56 

4. RESULTS AND DISCUSSION: COLLECTOR THERMAL PERFORMANCE AND EXPOSURE STUDIES 57 

4.1 Summary of Previously Reported Research Findings of This Program 57 

4.1.1 Collector Thermal Performance Test Data Uncertainty 57 

4.1.2 Analysis of Measurement and Calculation Procedures for Incident Angle 

Modifiers for Flat-Plate Solar Collectors 57 

4.1.3 Evaluation of Absorber Stagnation Temperature as an Indicator of 

Changes in Solar Collector Materials Performance 58 

4.1.4 Comparison of Solar Simulator and Outdoor ASHRAE Standard 93 Thermal 
Performance Tests 58 

4.2 Outdoor Slope and Intercept Data 59 

4.3 Analysis of Slope and Intercept Data 85 

4.3.1 Variation Within and Between Test Sites 88 

4.3.2 Influence of Experimental Apparatus 38 

4.3.3 Environmental Factors 95 

4.3.4 Effect on F R (xa) of Linearized Efficiency Curves 104 

4.4 Collector Thermal Performance Dependence on Material Properties 104 

4.5 Absorber Stagnation Temperature Measurement Results 10° 

4.5.1 General Considerations 1° 8 

4.5.2 Temperature H° 

4.5.3 Combined Irradiance and Ambient Temperature HO 

4.5.4 Use of Stagnation Temperature for Monitoring Changes in Thermal 

Properties HO 

4.6 General Observations on Outdoor Test Methods for Measurement of Collector 

Performance Degradation 115 



vn 



TABLE OF CONTENTS (Continued) 

Page 

5. CONCLUSIONS AND RECOMMENDATIONS 12 o 

5.1 Cover Materials 120 

5.2 Absorber Materials 121 

5.3 Solar Collectors 123 

6. REFERENCES 125 



APPENDIX A: SOLAR RADIATION EXPOSURE SUMMATIONS 



129 



Vlll 



LIST OF TABLES 

Page 

Table 2.1.1 Summary Description of Field Test Series for Solar Collectors 3 

Table 2.1.2 Test Collector Specimen Description 5 

Table 2.2.1 Exposure Tests for Cover Materials 7 

Table 2.2.2 Exposure Tests for Absorber Materials 7 

Table 2.2.3 Cover Test Materials 8 

Table 2.2.4 Absorber Test Materials 9 

Table 3.1.1 Transmittance of Full-Size Collector Cover Material After Exposure 19 

Table 3.1.2 Effect of "Real Time" Outdoor Exposure on Cover Sample Transmittance ..... 19 

Table 3.1.3 Effect of Accelerated Outdoor Tests on Cover Sample Transmittance 29 

Table 3.1.4 Effect of Temperature Exposure on Cover Sample Transmittance 29 

Table 3.1.5 Effect of Temperature and Humidity on Cover Sample Transmittance 



Table 3.2.8 Effect of Temperature on Absorber Coatings 

Table 3.2.9 Effect of Temperature and Humidity and Temperature and Xenon Arc Radiation 
on Absorber Coatings 



Table 4.3.1 Statistical Analysis of Efficiency Curve Intercept and Slope Parameters 
Based on Initial and Final Tests, All Collectors and Test Sites, 
Series 1 and 2 



36 



Table 3.1.6 Effect of Temperature and Xenon Arc Radiation on Cover Sample 

Transmittance 36 

Table 3.1.7 Effect of Environmental Exposure on the Molecular Weight of Poly(methyl 

methacrylate) 41 

Table 3.2.1 Absorptance of Full-Size Collector Absorber Coatings After Exposure 44 

Table 3.2.2 Emittance of Full-Size Collector Absorber Coatings After Exposure 45 

Table 3.2.3 Effect of Outdoor Exposure on Absorber Coating Absorptance-Selective 

Side 46 

Table 3.2.4 Effect of Outdoor Exposure on Absorber Coating Emittance-Selective Side .. 47 

Table 3.2.5 Effect of Outdoor Exposure on Absorber Coating Absorptance-Nonselective 

Side 49 

Table 3.2.6 Effect of Outdoor Exposure on Absorber Coating Emittance-Nonselective 

Side 50 

Table 3.2.7 Effect of Accelerated Outdoor Exposure on Absorber Coatings •• 51 



52 



54 

Table 3.2.10 Effect of Thermal Cycling on Absorber Coatings 55 

Table 4.2.1 Summary of all reported Fr(tci) and F^Ul Values 



60 



91 



Table 4.3.2 Residual Standard Deviations for Collector B, Test Series 1, Linear 

Efficiency Curve Correlations 



ix 



LIST OF TABLES (Continued) 



Page 



Table 4.3.3 Comparison of Frequency of Extreme Values for F^(xa) Reported by Test 

Sites 92 

Table 4.3.4 Correlations of Pyranometers and Test Stands with Experimental Results 
Outside Probable Measurement Uncertainty Ranges, Site 1, All Series 
through 120-day Retests 93 

Table 4.3.5 Reference Environment Used in Compensation for Environmental Dependence of 

Reported Results 96 

Table 4.4.1 Base Case Collector Parameters for Calculating Thermal Performance 

Dependence on Material Properties 107 

Table 4.4.2 Effects of Material Property Changes on Fr(toi) and FrUl for Collector D .. 108 

Table 4.4.3 Effects of Material Property Changes on F R (xa) and F R U L for Collector H .. 109 

Table 4.5.1 Hours of Exposure with Conditions Exceeding Corresponding Ambient 

Temperature and Irradiance Levels, Site 1, Phoenix, Arizona, July 1979 to 

June 1980 113 

Table 4.5.2 Hours of Exposure with Conditions Exceeding Corresponding Ambient 

Temperature and Irradiance Levels, Site 2, Cape Canaveral, Florida, 

November 1979 to October 1980 113 

Table 4.5.3 Hours of Exposure with Conditions Exceeding Corresponding Ambient 
Temperature and Irradiance Levels, Site 3, Palo Alto, California, 
July 1979 to June 1980 H 4 

Table 4.5.4 Hours of Exposure with Conditions Exceeding Corresponding Ambient 
Temperature and Irradiance Levels, Site 4, Gaithersburg, Maryland, 
April 1979 to March 1980 I 14 

Table A.l Exposure Summation for All Collectors 131 

Table A. 2 Exposure Summation for Material Samples at Phoenix J -33 

Table A. 3 Exposure Summation for Material Samples at Cape Canaveral -* 

Table A. 4 Exposure Summation for Material Samples at Palo Alto 13-> 

Table A. 5 Exposure Summation for Material Samples at Gaithersburg 13° 



LIST OF FIGURES 



Figure 3.1.2 Spectral transmit tance curves for cover samples from full-size collectors 
after 480 exposure days (^-17,000 kJ/sq-m-day) 



Figure 3.1.4 Spectral transmittance curves for cover samples exposed to "real time" 

and accelerated outdoor exposure on mini-boxes 

Figure 3.1.5 MLcrocracking of cover samples after accelerated exposure on mini-boxes 

equivalent to » 2 years "real time" 

Figure 3.1.6 Blistering of cover material H after 2000 h at 125°C 

Figure 3.1.7 Spectral transmittance curves of cover samples after temperature aging .. 



Figure 3.1.9 Spectral transmittance curves of cover samples after temperature and 
humidity aging 

Figure 3.1.10 Spectral transmittance curves of cover samples after temperature and 
xenon arc radiation aging 



Figure 4.2.1 Collector A, normalized Fr(toi) vs retests, series 1 and 2, all test 
sites , 

Figure 4.2.2 Collector B, normalized Fr(toO vs retests, series 1 and 2, all test 
sites 



Figure 4.2.4 Collector D, normalized Fr(toi) vs retests, series 1 and 2, all test 
sites , 



Page 
10 



Figure 2.2.1 Cover exposure mini-box 

Figure 2.2.2 Accelerated exposure cover mini-box H 

Figure 2.2.3 Accelerated exposure test plate " 

Figure 2.2.4 Outdoor absorber exposure box 

Figure 2.2.5 Accelerated exposure absorber mini-box 



13 

14 



Figure 3.1.1 MLcrocracking of cover samples from full-size collectors after 480 

exposure days (^-17,000 kJ/sq-m-day) ■*■" 



20 



Figure 3.1.3 MLcrocracking of cover samples after "real time" exposure on mini-boxes 

for 480 exposure days (^17,000 kJ/sq-m-day) 2 



24 

28 
31 
32 



Figure 3.1.8 Surface changes in cover material H after 500, 1000, and 2000 h at 90°C 

and 95% RH 35 



37 



40 



Figure 3.2.1 Whitish deposits on absorber material A sample from a full-size collector 

after 480 exposure days (^17,000 kJ/sq-m-day) 43 

Figure 3.2.2 Corrosion of absorber material D sample from a full-size collector after 

480 exposure days (^17,000 kJ/sq-m-day) 43 



61 



62 



Figure 4.2.3 Collector C, normalized Fr(toi) vs retests, series 1 and 2, all test 

sites 63 



64 



Figure 4.2.5 Collector E, normalized Fr(toi) vs retests, series 1 and 2, all test 

sites 65 

Figure 4.2.6 Collector F, normalized Fr(toi) vs retests, series 1 and 2, all test 

sites 66 



xi 



LIST OF FIGURES (Continued) 



Page 



Figure 4.2.7 Collector G, normalized Fr(toi) vs retests, series 1 and 2, all test 

sites 67 

Figure 4.2.8 Collector H, normalized Fr(toi) vs retests, series 1 and 2, all test 

sites 68 

Figure 4.2.9 Collectors A and B, normalized Fr(toi) vs retests, series 3 and 4, 

test sites 1 and 2 69 

Figure 4.2.10 Collectors C and D, normalized Fr(to() vs retests, series 3 and 4, test 

sites 1 and 2 70 

Figure 4.2.11 Collectors E and F, normalized Fr(tci) vs retests, series 3 and 4, test 

sites 1 and 2 '1 



Figure 4.2.12 Collectors G and H, normalized Fr(toc) vs retests, series 3 and 4, 

test sites 1 and 2 , 

Figure 4.2.13 Collector A, normalized FrUl vs retests, series 1 and 2, all test 

sites 

Figure 4.2.14 Collector B, normalized FrUl vs retests, series 1 and 2, all test 

sites , 

Figure 4.2.15 Collector C, normalized FrUl vs retests, series 1 and 2, all test 

sites , 

Figure 4.2.16 Collector D, normalized FrUl vs retests, series 1 and 2, all test 

sites 

Figure 4.2.17 Collector E, normalized FrUl vs retests, series 1 and 2, all test 

sites , 

Figure 4.2.18 Collector F, normalized FrUl vs retests, series 1 and 2, all test 

sites , 

Figure 4.2.19 Collector G, normalized FrUl vs retests, series 1 and 2, all test 

sites 

Figure 4.2.20 Collector H, normalized FrUl vs retests, series 1 and 2, all test 

sites , 

Figure 4.2.21 Collectors A and B, normalized FrUl vs retests, series 3 and 4, test 
sites 1 and 2 , 

Figure 4.2.22 Collectors C and D, normalized FrUl vs retests, series 3 and 4, test 
sites 1 and 2 , 



72 



73 



74 



75 



76 



77 



78 



79 



80 



81 



82 



Figure 4.2.23 Collectors E and F, normalized FrUl vs retests, series 3 and 4, test 

sites 1 and 2 83 

Figure 4.2.24 Collectors G and H, normalized FrUl vs retests, series 3 and 4, test 

sites 1 and 2 84 

Figure 4.3.1 Collector B, aggregate plot of measured efficiency, series 1, test 

site 1, all retests 86 

Figure 4.3.2 Collector E, aggregate plot of measured efficiency, series 1, test 

site 1, all retests 86 

Figure 4.3.3 Collector F, aggregate plot of measured efficiency, series 1, test 

site 1, all retests 87 

xii 



LIST OF FIGURES (Continued) 



Figure 4.3.7 Collector B, efficiency curve slope parameters vs exposure time, all test 
sites, series 1 



Figure 4.3.9 Comparison of calculated and measured efficiencies for collector B, 
site 1, test series 1, all retest data , 



Figure 4.3.11 Values of FrUl for collector B, site 4, test series 1 vs exposure time, 

adjusted to standard test conditions 

Figure 4.3.12 Reported values of Fr(toi) for collector B, site 4, test series 1 vs 

exposure time 

Figure 4.3.13 Values of Fr(toi) for collector B, site 4, test series 1 vs exposure time, 
adjusted to standard test conditions 

Figure 4.3.14 Reported values of FrUl for collector B, sites 1, 2, and 3, test series 1 
vs exposure time 



Figure 4.3.16 Reported values of Fr(tcx) for collector B, sites 1, 2, and 3, test 
series 1 vs exposure time 



Page 



Figure 4.3.4 Collector B, efficiency curve intercepts vs exposure time, test site 1, 

all series 89 

Figure 4.3.5 Collector B, efficiency curve slope parameters vs exposure time, test 

site 1, all series 89 

Figure 4.3.6 Collector B, efficiency curve intercepts vs exposure time, all test 

sites, series 1 90 



90 



Figure 4.3.8 Typical pyranometer calibrations vs time 94 

97 



Figure 4.3.10 Reported values of FrUl for collector B, site 4, test series 1 vs 

exposure time 98 



98 



99 



99 



100 



Figure 4.3.15 Values of FrUl for collector B, sites 1, 2, and 3, test series 1 vs 

exposure time, adjusted to standard conditions 100 



101 



Figure 4.3.17 Values of Fr(toi) for collector B, sites 1, 2, and 3, test series 1 vs 

exposure time, adjusted to standard conditions 101 

Figure 4.3.18 Reported values of FrUl for collector E, sites 1, 2, and 3, test series 1 

vs exposure time 102 

Figure 4.3.19 Values of FrUl for collector E, sites 1, 2, and 3, test series 1 vs 

exposure time, adjusted to standard conditions 102 

Figure 4.3.20 Reported values of Fr(tcx) for collector E, sites 1, 2, and 3, test 

series 1 vs exposure time 103 

Figure 4.3.21 Values of F^(xa) for collector E, sites 1, 2, and 3, test series 1 vs 

exposure time, adjusted to standard conditions 103 

Figure 4.3.22 Comparison of measured F^(xa) to values extrapolated from linearized 

efficiency curves 105 

Figure 4.4.1 Calculated effects on thermal performance of material property changes 

for collectors D and H 106 

Figure 4.5.1 Typical absorber stagnation temperature profiles of exposed collectors, 

site 2, clear day Ill 



Xlll 



LIST OF FIGURES (Continued) 



Page 



Figure 4.5.2 Daily absorber stagnation temperature profiles for double-glazed 

collectors B and D, site 4, clear day Ill 

Figure 4.5.3 Comparison of absorber stagnation temperature profiles using solar 

irradiance simulators and outdoor exposure, collector D 112 

Figure 4.5.4 Days per year above minimum solar irradiance levels for each of the 4 

test sites 112 

Figure 4.5.5 Sensitivity of normalized absorber stagnation temperature to changes in 

material properties 116 

Figure 4.5.6 Effect of environmental test conditions on normalized absorber stagnation 

measurements 117 

Figure 4.5.7 Sensitivity of the all-day integration method to a 0.10 change in plate 

absorptance for collector B 118 



xiv 



1 . INTRODUCTION 

Public Law 93-409, the "Solar Heating and Cooling Demonstration Act of 1974" [1]* and Public 
Law 93-473, the "Solar Energy Research, Development and Demonstration Act" [2] authorized a vigorous 
Federal program of research, development, and demonstration to help establish solar energy as a 
viable energy resource for the nation. The primary goal of the program, as stated in the National 
Program for Solar Heating and Cooling of Buildings (ERDA 76-6) [3], is to work with industry in the 
development and early introduction of economically competitive and environmentally acceptable solar 
energy systems to help meet national energy requirements. 

The National Bureau of Standards (NBS) and other Federal agencies are cooperating with various 
private sector organizations in the development of consensus methods for the installation, perfor- 
mance, and testing of solar equipment. This effort has resulted in the publication of several 
procedures that can be used to assess the thermal performance and durability of solar energy systems 
and the components and materials used in their construction. The American Society of Heating, 
Refrigerating, and Air-Conditioning Engineers (ASHRAE) Standards 95-1981 [4], 94-77 [5], and 93-77 
[6] for testing the thermal performance of residential hot water systems, thermal storage devices, 
and solar collectors, respectively, were based to a considerable extent on technical data generated 
by NBS. Similarly, the American Society for Testing and Materials (ASTM) has developed several 
standards concerned with the reliability and durability of solar collectors [7], cover plate 
materials [8, 9, 10, 11], absorptive materials [12, 13], rubber hoses and seals [14, 15, 16, 17, 
18], metal-heat transfer fluid compatibility [19, 20], and polymeric containment material -heat 
transfer fluid compatibility [21]. 

In 1977, a program was initiated by NBS to help provide an experimental basis for the development of 
consensus standards for assessing the reliability and durability of solar collectors and their 
materials. In this program, eight different types of flat-plate solar collectors and small-scale 
cover and absorber materials specimens, representative of those in use at that time, were exposed 
outdoors at four sites located in different climatic regions. Periodic measurements were made of 
their performance as a function of exposure time. Laboratory aging tests were conducted concurrently 
on specimens of the same materials to provide a basis for comparison with the outdoor exposure tests. 

This report presents and discusses the results obtained for these outdoor and laboratory aging tests. 
Several additional publications resulted from this program. These include: 

• an overall program test plan [22, 23]; 

• an analysis of thermal performance data uncertainty for liquid-heating flat-plate solar 
collectors [24, 25]; 

• a discussion of the determination of incident angle modifiers for flat-plate solar 
collectors [26, 27]; 

• a comparison of outdoor and solar simulator solar collector thermal performance tests [28]; 

• a discussion of preliminary test program results [29]; 

• an evaluation of the use of absorber stagnation temperature as a parameter for determining 
changes in flat-plate solar collector performance [30, 31]; and 

• an analysis of an integrated day-long stagnation temperature technique for detecting changes in 
solar collector performance [32]. 

Key research findings contained in these publications are discussed in section 4.1 of this report. 

This work is intended to be complementary to other collector materials test methods development 
projects underway at NBS and elsewhere by providing a basis for comparing the results of small-scale 
materials level tests with changes taking place in full-size commercially available solar collectors. 

Throughout this report, reference is made to days of outdoor exposure. Unless otherwise specified, 
the term "days" refers to days with solar radiation levels of 17,000 kJ/m^ (1500 Btu/ft^) or greater 
measured in the plane of the test specimen. These 17,000 kJ/m^ days were used for accounting pur- 
poses rather than calendar days in an attempt to make the solar radiation exposure at the various 
outdoor exposure test sites more comparable. 



* Numbers in brackets indicate references given in section 6 of this report, 



2. TEST PROGRAM OVERVIEW 

The tests and exposure procedures described herein are intended to determine the influence of 
environmental exposure parameters that could affect the degradation of solar collectors and their 
materials. They are also intended, to the extent possible, to provide a correlation between changes 
that occur at the materials and the collector component levels. 

A more complete description of the test procedures summarized in this section is given in NBS 
Technical Note 1136 [22]. 

2.1 SOLAR COLLECTOR TESTS 

2.1.1 Outdoor Exposure Conditions 

The four different collector test series selected for use in the program and the purpose of each test 
series are summarized in table 2.1.1. 

Series 1 and 2 were intended to evaluate the effects of "normal" stagnation conditions. The 
collectors in series 1 were allowed to stagnate dry, whereas the collectors in series 2 were filled, 
allowed to stagnate under filled conditions with a pressure relief valve set to the maximum allowable 
collector pressure, and subjected to thermal shock/cold fill and thermal shock/water spray tests at 
specified time intervals. The purpose of exposing filled collectors to stagnation conditions was to 
evaluate the combined effects of the temperatures and pressures that would occur under stagnation 
conditions. 

Series 3 was intended to determine whether or not changes in collector performance will occur under 
the reduced absorber plate temperatures characteristic of operational conditions. Augmentation 
reflectors were used to amplify the solar radiation to which stagnating collectors were exposed in 
series 4. This series was intended to determine the effects that such reflectors would have when 
they are used in actual systems and to determine whether or not reflectors are a practical way of 
accelerating the degradation of stagnating collectors. 

These exposure conditions were intended to subject the solar collectors and their materials to 
different thermal stress levels at each outdoor exposure site. The thermal stress level that would 
occur in normal use most likely lies between the series 1 and 2 and the series 3 test conditions, 
with some stagnation being a normal occurrence. 

The stagnation testing of solar collectors is often considered to be an accelerated test in that it 
exposes solar collectors to temperatures that would not occur with the heat transfer fluid flowing. 
However, solar collectors may frequently be exposed to stagnation conditions in normal service. 
This can occur either when the collectors are initially installed, before system start-up, or when 
the system is shut down for maintenance or for seasonal considerations. In commercial buildings, 
time periods of up to a year between the installation and start-up of equipment have been 
experienced. Thus, only that portion of stagnation exposure time which would not be attributed to 
normal service can be considered to represent accelerated aging. 

Four outdoor exposure test sites were selected which represent both the median and extreme United 
States climatological conditions. These test sites can be briefly described as follows: 

Climatological Extremes 

Site 1. hot, dry 

high solar radiation 

(high UV radiation would accompany these conditions) 

The hot, dry condition can be found in southwestern states (i.e., Arizona, Nevada, and New Mexico). 
DSET Laboratories, Inc., located in Phoenix, Arizona was selected as this test site. 

Site 2. hot, humid 

high solar radiation 

(low to moderate UV radiation would accompany these conditions) 

The hot, humid condition can be found either in Florida or along the Gulf Coast in the states of 
Alabama, Mississippi, Louisiana or Texas. The Florida Solar Energy Center, located in Cape 
Canaveral, Florida was selected as this test site. 



Table 2.1.1 Summary Description of Field Test Series for Solar Collectors 





Collector 


Conditions 




Test 


Performance 


for Weathering 


Purpose of 


Series 


Measurement 


Exposure 


Test Series** 


Series 1 


Initial measurement in 


Each collector preconditioned 


1. Observation of effects 


"dry stagnation" 


accordance with ASHRAE 


for each weathering exposure 


of dry stagnation 




93-77 except delete 


by purging with dry air to 


collector performance 




3 day pre-exposure and 


remove the remaining heat 


and other characteris- 




measurement of time 


transfer fluid. Successive 


tics for various 




constant 


weathering exposure between 
performance retests provide 


weathering times. 




Performance retest 


cumulative exposures of 3, 


2. Provide data for compar- 




after 3, 15, 30, 60, 


15, 30, 60, 120, 240, and 


ing initial performance 




120, 240 and 480 day 


480 days.* 


without 3 day pre-expo- 




exposures.* 




sure per ASHRAE 93-77. 


Series 2 


Initial measurement in 


Collectors preconditioned for 


1. Observation of effects 


"no-flow 


accordance with ASHRAE 


weathering exposure by filling 


of no-flow stagnation on 


stagnation" 


93-77 after 3 day pre- 


per NBSIR 78-1305A [33], 


collector performance and 




exposure. Delete 


capping and allowing to boil 


other characteristics. 




measurement of time 


dry. Weathering exposures 






constant. 


same as in series 1 except 


2. Observation of effects 






that Thermal Shock Tests per 


of Thermal Shock Tests 




Performance retests 


NBSIR 78-1305A will be per- 


representing: (a) filling 




same as in series 1 . 


formed during first 30 day 


a hot collector with cool 






exposure on series 2 test 


heat transfer medium, 






collectors only. 


and (b) rain on a hot 
collector. 

3. Observation of static 
pressure leakage after 
30 and 120 days of 
exposure 


Series 3 


Performance of test 


During weathering exposure, 


1. Observation of effects of 


"controlled 


collectors measured in 


heat transfer flow rate main- 


normal operation on col- 


flow" 


accordance with ASHRAE 


tained at 25% of operational 


lector performance and 




93-77 , taking only 3 


flow rate for liquid. 


other characteristics. 




points. Delete 3 day 








pre-exposure and time 








constant measurement. 








Performance retests 








up to 240 days, 








same as series 1 . 






Series 4*** 


Initial measurement 


Preconditioning and weathering 


1. Observation of effects of 


"dry stagnation 


same as in series 1 . 


exposures same as in series 1 , 


dry stagnation on collec- 


with augmenta- 




except that a reflector was 


tor performance and other 


tion reflectors" 


Performance retests 


used on each collector 


characteristics with 




same as in series 1 . 


during each day of weathering 


solar radiation amplified 






exposure.* Solar radiation 


by a reflector. 






measurements required both 








with and without reflector. 


2. Obtaining temperature 

history within collectors 
for most severe exposure 
conditions. 



* Individual days with solar radiation of 17,000 kj/m 2, day or greater as measured in the plane of the 
collector aperture without the influence of a reflector. 

** All series include provision of data for comparisons between test series, test sites (climatic 
regions), collector designs, etc. 



*** This series was terminated prematurely after 60 days* due to a serious lack of uniformity in 
radiation caused by the augmentation reflectors. 



Median Climatological Conditions 

Site 3. moderate temperature 
dry 

high solar radiation 
moderate UV radiation 

The moderate, dry condition can he found primarily in parts of California. The Lockheed Research 
Laboratory, located in Palo Alto, California was selected as this test site. 

Site 4. moderate temperature 
humid 

moderate solar radiation 
moderate to low UV radiation 

The moderate, humid condition can be found in the Pacific Northwest, Midatlantic and Midsouth regions 
of the United States. The NBS test facility, located in Gaithersburg, Maryland served as this test 
site. 

All four test series, each having a sample of the eight collector types described in section 2.1.2, 
were conducted at sites 1 and 2. Only series 1 and 2 were conducted at sites 3 and 4. 

2.1.2 Collector Description 

Eight types of liquid-heating flat-plate type solar collectors were selected for use in the test 
program. The designs chosen were representative of commonly used materials and types of construction. 
All collectors from each manufacturer were from the same production lot. The collector cover and 
absorber materials, their pertinent optical properties, and average collector areas (gross and 
aperture) are listed in table 2.1.2. The absorber material optical property data are based upon 
measurements of at least ten samples taken from an actual absorber of each collector type prior to 
aging. The solar transmittance of the glass materials was obtained using the full cover and a 
pyranometer (ASTM Standard E 424, Method B [34]). The solar transmittance of nonglass cover materials 
and the solar absorptance values were obtained from spectral measurements with an integrating sphere 
(ASTM Standard E 424, Method A [34]). Emittance was measured using a portable-type instrument 
employing a thermopile and infrared reflectance technique, in accordance with ASTM Standard E 408, 
Method A [35] . 

Detailed descriptions of each collector's construction dimensions and pertinent material properties 
useful for thermal analytical modeling are listed in Appendix B of NBS Technical Note 1140 [25]. 
Type T thermocouples were attached to the center of the underside of the collector absorber plates 
for the measurement of stagnation temperatures. 

Collectors were identified by a coding scheme which will be used in later sections of this report in 
conjunction with test data. In this scheme, a letter identifying the collector type (A through H) 
is followed by a number identifying the test site (1 to 4), which in turn is followed by another 
number identifying the test series (1 to 4). For example, G14 represents collector type G exposed 
at test site 1 to the series 4 exposure conditions. 

2.1.3 Measurements and Observations 

Thermal performance measurements were made on the collectors in each test series in accordance with 
the schedule summarized in table 2.1.1. The ASHRAE Standard 93-77 thermal performance test procedure 
[6] was used in this program both as a full test (four temperatures, four data points at each temper- 
ature) and as a three-point performance retest (three temperatures, four data points at each tempera- 
ture). The retest was conducted to demonstrate the magnitude of changes in the intercept and slope 
of the efficiency curve as a function of environmental exposure time. The measurements were required 
to be spread over the range of 0.02 to 0.07 o C*m2/W. The term "performance retest" will be used 
throughout this document when the three-point test is specified. 

Data collected in addition to collector thermal performance included the following: 

• Key environmental parameters; high and low daily ambient temperature, total and diffuse solar 

radiation, peak hourly solar radiation, wind velocity, precipitation, and visual sky and weather 
conditions, on a daily basis. 



Table 2.1.2 Test Collector Specimen Description 



Collector 
Code 


Cover Material 


Absorber Material 


Average Areas* 


Outer 


Inner 


Solar 
Transmittance 


Material 


Solar 
Absorptance 


Emittance 


Gross 
(m 2 ) 


Aperture 
(m 2 | 


A 


Water White 
Glass 





0.90 


Black Nickel 


0.87 


0.13 


2.150 


1.831 


B 


Low Iron 
Glass 


Low Iron 
Glass 


0.88 (ea) 


Black Velvet 
Paint 


0.97 


0.96 


1.732 


1.602 


C 


Plate Glass 


Thin Film 
Heat Trap 


0.86 


Black Velvet 
Paint 


0.98 


0.92 


2.589 


1.924 


D 


Etched 
Glass 


Etched 
Glass 


0.96 (ea) 


Black Chrome 


0.97 


0.07 


1.655 


1.402 


E 


FRP** 
(Type I) 





0.85 


Lacquer 
Primer 


0.95 


0.87 


1.892 


1.720 


F 


Water White 
Glass 





0.90 


Copper Oxide 


0.96 


0.75 


1.922 


1.769 


G 


FRP** 
(Type II) 


FEP*** 
Film 


0.84 ** 
0.96 *** 


Porcelain 
Enamel 


0.93 


0.86 


2.563 


2.188 


H 


PET**** 
Film 


pgp*** 
Film 


0.85 **** 
0.96 *** 


Siliconized 

Polyester 

Paint 


0.95 


0.89 


2.916 


2.641 



* Average of values reported by the four test sites. 

** Glass fiber reinforced plastic 

*** Fluorinated (ethylene propylene) copoylmer. 

**** Poly(ethylene terephthalate) 



• Maximum daily collector absorber plate temperature (for all collectors). 

• Daily profiles of absorber plate temperature (for collectors D and H) , ambient temperature, 
irradiance, and wind velocity at all four outdoor exposure sites for a period of one year. 

• Visual observations of changes in collector appearance during outdoor exposure and of 
disassembled collectors following the completion of outdoor exposure and thermal performance 
testing. 

• Optical property measurements on coupon samples of polymeric cover materials and absorber 
materials taken from disassembled exposed and unexposed collectors and microstructural studies 
on specimens showing degradation. 

2.2 COLLECTOR MATERIALS TESTS 

2.2.1 Exposure Conditions 

Coupon specimens of cover plate and absorber materials were subjected to several different types of 
laboratory and outdoor environmental exposure tests. These specimens consisted of samples taken 
from the eight types of full-size collectors used in the test program and of several additional 
materials of interest. Changes in the optical properties of these materials were measured as a 
function of exposure time. Microstructural studies were conducted on materials showing visible 
degradation. 

Outdoor exposure conditions at the materials coupon specimen level included "real time" exposure in 
simulated collectors and exposure to concentrated radiation in machines, described in ASTM E 838 
[36]. Indoor laboratory tests conducted included exposure to: (1) temperature, (2) combined temper- 
ature and humidity, (3) combined temperature and radiation, and (4) thermal cycling (absorber materials 
only). Additional materials level exposure tests were conducted in xenon arc and tungsten lamp solar 
simulators. 

The outdoor "real time" materials level exposures were conducted concurrently with those on 
full-scale collectors at the four outdoor exposure test sites discussed in section 2.1.1. 

The exposure conditions used for the cover and absorber materials are summarized in tables 2.2.1 and 
2.2.2, respectively. The exposure conditions summarized in these tables are intended to simulate a 
broad range of environmental stress conditions. Primary emphasis was placed on exposure to temper- 
ature, solar radiation, and moisture. Other degradation factors such as hail, pollutants, and dust 
are localized in nature and were assessed via their occurrence at the "real time" outdoor exposure 
test sites participating in the program. 

2.2.2 Test Specimens 

Materials level tests were conducted on the cover and absorber materials listed in tables 2.2.3 and 
2.2.4. Materials specimens having code letters "A" through "H" were cut from solar collectors of 
the same type and batch as those exposed outdoors as full-size collectors. Figures 2.2.1 through 
2.2.5 depict the apparatus in which test specimens were mounted for outdoor exposure. 

2.2.3 Measurements and Observations 
Data collected included: 



Integrated solar transmittance and absorptance per ASTM E 424, Method A. 

Emlttance per ASTM E 408, Method A. 

Normal and hemispherical spectral transmittance curves measured on spectrophotometers with 
and without integrating spheres. 

Visual observation of changes. 

Microscopic evaluations using both optical and scanning electron microscopes. 

Key environmental parameters, as for solar collector exposure. 

Test apparatus exposure temperatures on a daily basis. 



Table 2.2.1 Exposure Tests for Cover Materials 



Exposure Condition 


Value or Range 


Exposure Time 


Temperature (indoor) 


a) 70°C 

b) 90°C 

c) 125°C 


500, 1,000, and 2,000 h 


Temperature and Humidity (indoor) 


a) 70°C and 95% RH 

b) 90°C and 95% RH 


500, 1,000, and 2,000 h 


Temperature and Radiation (indoor) 


Xenon arc weathering 
machine 

a) 70°C 

b) 90°C 


500, 1,000, and 2,000 h 


Solar Simulator 


a) Tungsten 

b) Xenon simulators 
with irradiance 
of ~ 950 W/m 2 and 
~ 70°C 


30, 60, and 120 cycles* 


"Real Time" Outdoor 


1 sun at ~ 60°C 


80, 160, 240, and 480 
days** 


Accelerated Outdoor 


~ 6 suns at ~ 70°C 


6, 12, and 24 equivalent 
months*** 



* Each cycle consists of 5 h irradiation and 1 h cooling 
** Days having a minimum radiant exposure of 17,000 kj/m 2 . 
*** One equivalent month equals 6.625 x 10° J/m 2 (15,835 Langleys) 



Table 2.2.2 Exposure Tests for Absorber Materials 



Exposure Condition 


Value or Range 


Exposure Time 


Temperature (indoor) 


a) 150°C 

b) 175°C 


1,000 and 2,000 h 


Temperature and Humidity (indoor) 


90°C and 95% RH 


1,000, and 2,000 h 


Thermal Cycling (indoor) 


-10°C to 175°C 


5, 15, and 30 cycles 


Temperature and Radiation (indoor) 


Xenon arc weathering 
machine at 90°C 


1,000 and 2,000 h 


Solar Simulator 


a) Tungsten 

b) Xenon simulators 
with irradiance 
of ~ 950 W/m 2 and 
~ 130°C 


30, 60, and 120 cycles* 


"Real Time" Outdoor 


1 sun at ~ 140° C and 
~ 160°C 


80, 160, 240, and 480 
days** 


Accelerated Outdoor 


~ 6 suns at ~ 150°C 


6, 12, and 24 equivalent 
months*** 



* Each cycle consists of 5 h irradiation and 1 h cooling 
** Days having a minimum radiant exposure of 17,000 kj/m 2 . 
*** One equivalent month equals 6.625 x 10** J/m 2 (15,835 Langleys). 



Table 2.2.3 Cover Test Materials 



Code 1 


Cover Material 


Solar 
Transmit tance 


(Controls) 


E 


FRP 4 Type la 


0.85 




G 


FRP 4 Type II 


0.84 




H 2 


PET 5 / FEP 6 
(outer) / (inner) 


0.85/0.96 




J 


Polycarbonate 


0.88 




K 


Poly(vinyl fluoride) 


0.89 




L 


FRP 4 Type lb 


0.84 




M 


FRP 4 Type III 


0.78 




N 


Poly(methyl methacrylate) 


0.90 




2 


Glass 7 / Poly(vinyl fluoride) 
(outer) / (inner) 


0.86/0.89 





1 Code letters E, G, and H indicate materials coupon specimens cut from 
solar collectors E, G, and H. Codes, J, K, L, M, N, and tested at 
the materials level only. 

2 Materials exposed as a combination in the cover mini-boxes and in the 
accelerated exposure cover mini-boxes. Materials exposed individually 
in all other tests. Glass and FEP materials were not exposed 
individually. 

3 These properties are dependent on the formulation and manufacturing 
processes used. Other products within a generic class of materials 
may have significantly different properties. 

* Glass fiber reinforced plastic. 

5 Poly(ethylene terephthalate) . 

6 Fluorinated (ethylene propylene) copolymer. 
' Ordinary plate glass. 



Table 2.2.4 Absorber Test Materials 



Code 1 


Absorber Material 


Optical Properties 2 


Coating 


Substrate 


Solar 
Absorptance 


Emittance 3 


A 


Black Nickel 


Steel 


0.87 


0.13 


C 


Black Velvet Paint 


Copper 


0.98 


0.92 


D 


Black Chrome 


Steel (nickel 
flashed) 


0.97 


0.07 


E 


Black Lacquer Primer 


Copper 


0.95 


0.87 


F 


Copper Oxide 


Copper 


0.96 


0.75 


G 


Black Porcelain 
Enamel 


Steel 


0.93 


0.86 


H 


Black Siliconized 
Polyester Paint 


Aluminum 


.-. 0.95 


0.89 


I 


Black Chrome 


Stainless Steel 


0.88 


0.19 


J 


Black Chrome 


Aluminum 


0.98 


0.14 


L 


Lead Oxide 


Copper 


0.99 


0.29 


M 


Oxide Anodized 


Aluminum 


0.94 


0.10 


N 


Oxide Conversion 
Coating 


Aluminum 


0.93 


0.51 


P 


Black Chrome 


Copper (nickel 
flashed) 


0.96 


0.08 



1 Code letters A through H indicate materials coupon specimens cut from solar 
collectors A through H. Codes I through P tested at the materials level only. 

2 These properties are dependent on the formulations and manufacturing processes 
used. Other products within a generic class of materials may have significantly 
different properties. 

3 Average values based on a minimum of ten test specimens. 




RAW SHIELD 

OUTER COVER 
TEST SPECIMEN 

SPACER FRAME 

MNER COVER 
TEST SPECIMEN 

SILICONE RUBBER 
SEALANT 

STAINLESS STEEL 
PAN 



BSORBER PLATE 
E T THERMOCOUPLE 
GLASS FIBER INSULATION 



Notes: 



Stainless Steel Pan: 22 x 12 x 10 cm without rim 

Glass Fiber Insulation: 64 kg/iiP density 

Bottom thickness 10 cm 

Edge thickness 2.5 cm wide x 2.5 cm thick 

Baked out at 230°C for 24 hours 

Absorber Plate: Black chrome on copper 

Silicone Rubber Sealant: Between covers and pan, and covers and spacer 

Cover Test Specimens: 26 x 16 cm 

Rain Shield: 16 ga stainless steel, clamped to pan 

Spacer Frame: 6 mm thick aluminum 



Figure 2.2.1 Cover exposure mini-box 



10 



COVER TEST SPECIMEN 

SILICONE RUBBER SEAL 



ALUMINUM PAN 





TYPE T THERMOCOUPLE 



ALUMINUM FOIL ABSORBER 



Notes: 

Cover Size: 7.6 x 5.1 cm 

Aluminum Pan: 11.5 x 6.5 x 2 cm 

Aluminum foil painted black and baked at 230°C for 24 hours 



Figure 2.2.2 Accelerated exposure cover mini-box 



11 




ALUMINUM PLATE 
310><140mm (painted black 
& baked @ 230°C for 
24 hrs.) 

COVER TEST SPECIMEN 
51 x 38mm 



ii i t 



CERAMIC INSULATOR 
12.7mm long 



TYPE T THERMOCOUPLE 



Figure 2.2.3 Accelerated exposure test plate 



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13 



FILM COVER 



ABSORBER TEST SPECIMEN 



ALUMINUM PAN 




TIEDOWN WIRE 



TYPE T THERMOCOUPLE 



Notes: 



Film Cover: Polytetraf luoroethylene 
Absorber Test Specimen: 7.5 x 5.1 cm 
Aluminum Pan: 11.5 x 6.5 x 2 cm 

Interior painted black and baked at 230°C for 24 hours 



Figure 2.2.5 Accelerated exposure absorber mini^box 



14 



3. RESULTS AND DISCUSSION: MATERIALS EXPOSURE STUDIES 

The sections which follow present results obtained through the outdoor and indoor laboratory testing 
of solar collector cover and absorber materials. Additional materials-related observations made 
during the disassembly of full-size solar collectors following outdoor exposure testing are also 
presented. The properties of materials discussed in this section are dependent on the formulations 
and manufacturing processes used. Other products within a generic class of materials may have 
significantly different properties. 

3.1 COVER MATERIALS TESTING 

In this section, the results of outdoor and indoor laboratory exposure testing of cover materials are 
presented and the extent to which accelerated outdoor and indoor laboratory tests simulate "real 
time" outdoor exposure is discussed. Materials used for these comparisons included: poly(ethylene 
terephthalate) - material code H, polycarbonate - material J, poly(vinyl fluoride) - material K, 
poly(methyl methacrylate) - material N, and several types of glass fiber reinforced plastics - materials 
E, G, L, and M. Materials E and L were essentially the same, with the exception of specimen thick- 
ness. Fluorinated (ethylene propylene) copolymer film was also investigated as an inner glazing, 
however, it is not discussed in detail in this section since there were no obvious changes observed 
in any of the exposure tests performed. The same is true for glass materials used as an outer glazing. 
In addition to the small-scale tests which were performed on all materials, samples of cover materials 
E, G, and H were cut from full-size solar collectors having the same code letter designations and 
evaluated following the completion of collector exposure testing as described in section 2 of this 
publication. The types of small-scale outdoor and indoor laboratory tests performed on cover materials 
are also summarized in section 2. The reader should note that visual observations of yellowing in 
materials E and L are somewhat biased towards the low side since these materials contained a blue 
additive which could conceal a considerable amount of yellowing on visual inspection. 

Both hemispherical and normal spectral transmittance measurements were made in the UV-visible range 
using spectrophotometers with and without integrating spheres, respectively . The integrating sphere 
data are representative of the amount of radiation that would reach a flat-plate solar collector 
absorber surface. The normal measurements made without an integrating sphere are a more sensitive 
indicator of changes in scattering, especially in materials which do not contain reinforcing fibers. 
Integrated solar transmittance values given in the tables contained in this section were calculated 
in accordance with ASTM E 424, method A using the air mass 2 solar spectrum. 

3.1.1 Full-Size Collector Stagnation Testing 

Visual Inspection : Cover material E showed a slight amount of yellowing after "real time" 
exposure for 480 exposure days at all four outdoor exposure sites. There was also evidence of minor 
surface dulling at the Cape Canaveral and Palo Alto sites. A moderate amount of yellowing was 
observed for material G after 480 days at all four sites. In addition, the exposed surfaces of this 
glazing became quite dull and readily absorbed liquids such as ink and there was some resin erosion 
which exposed glass fibers. Cover material H showed a considerable amount of top surface dulling 
after "real time" exposure for 480 days at all four outdoor exposure sites. The material also became 
quite brittle as a result of this exposure and snapped readily when it was bent so that the exposed 
surface was in tension. It was much more resistant to breaking when the bottom (unexposed) surface 
was in tension on flexing. The covers on several of the H collectors tore from the stresses imposed 
when the collectors were moved from exposure racks to test stands for periodic thermal performance 
measurements and had to be patched. In addition, the covers of all of the H collectors on exposure 
at the Phoenix test site were punctured in several places when exposed to impact from 1.2 cm maximum 
diameter hailstones. Testing previously conducted at NBS on unaged material indicated that this 
unaged material was capable of withstanding impact by 2.5 cm hailstones without puncture. 

Microscopic Examination : Examination of the exposed surfaces of materials G and H with optical and 
scanning electron microscopes revealed that the surface dulling was primarily due to the formation 
of micro-cracks (see figure 3.1.1). The microcracking was more extensive at the Cape Canaveral and 
Palo Alto sites than at the Phoenix and Gaithersburg sites for both materials. There was evidence 
of directional stresses in the microcrack patterns observed for material H. A minor amount of 
surface pitting was observed optically on those material E specimens which showed surface dulling. 
The same type of microcracking observed in this program for material H was also observed by the 
authors in samples of the same type of material taken from solar collectors that were installed on a 
building in Colorado for about 5 years. 



15 




, 50 /u m | 




(a) Material H - Phoenix 



(b) Material H - Cape Canaveral 





(c) Material H - Palo Alto 



(d) Material H - Gaithersburg 



Figure 3.1.1 Microcracking of cover samples from full-size collectors after 
480 exposure days (5-17,000 kj/sq-m-day) 



16 




(e) Material G - Cape Canaveral 




(f) Material G - Gaithersburg 



Figure 3.1.1 continued 



17 



Optical Measurements : Integrated solar transmlttance values are given in table 3.1.1 for cover 
samples cut from full-size collectors E, G, and H following exposure for 480 days at the four test 
sites. Material E had minor changes that are probably due to variations in sample homogeneity. 
Material G showed a substantial amount of change, with the greatest changes occurring at the Cape 
Canaveral site and the least at the Phoenix site. Transmittance curves for samples of materials G 
and H cut from the series 2 collectors at all four sites are shown in figure 3.1.2. The changes 
shown in the curves for material G are most likely due to a combination of yellowing (evidenced by 
the shift in the absorption edge* of the curve to longer wavelengths) and the extent of micro- 
cracking, which was greater at the Cape Canaveral site than at the Phoenix and Gaithersburg sites. 
Material H showed the greatest changes at the Cape Canaveral and Palo Alto sites. Examination of 
the transmittance curves for samples of material H taken from series 2 collectors exposed at all four 
sites and scanning electron microscope photos (see figure 3.1.1) shows that these changes were most 
likely due to the extent of microcracking, which was greatest at the Cape Canaveral and Palo Alto 
sites . 

3.1.2 "Real Time" Outdoor Mini-Box Testing 

Visual Inspection : Outdoor "real time" exposure of materials E and L on cover mini-boxes caused 
slight yellowing after 480 days at the Cape Canaveral and Palo Alto sites with no visual signs of 
degradation after 480 days at the Phoenix or Gaithersburg sites or at shorter exposure times at any 
of the sites. A slight to moderate amount of yellowing and exposed surface dulling was observed for 
material G after 480 days at all four sites with the most severe changes at the Cape Canaveral and 
Palo Alto sites. Exposure for 160 and 240 days caused slight yellowing at the Cape Canaveral, 
Phoenix, and Palo Alto sites, but no visual evidence of surface dulling. A slight increase in 
yellowing and a minor loss in surface gloss was also observed for material M after 480 days. 
Exposure of material H caused top surface dulling and embrittlement after 480 days at all four sites, 
but not after shorter exposure times. In addition, the same hail damage noted in the previous sec- 
tion for full-size collectors also occurred for the material H mini-box specimens at Phoenix. Very 
slight yellowing was observed in material J after 80 days at all four sites which progressed to 
slight to moderate after 480 days. In addition, surface dulling was observed after 240 and 480 days at 
all four sites. Exposure at the Palo Alto and Cape Canaveral sites caused noticeably more severe 
changes. With the exception of embrittlement after 480 days at the Phoenix site, there were no 
other visible signs of degradation of material K. Outdoor "real time" exposure of material N caused 
a minor amount of yellowing and a noticeable increase in embrittlement on breaking in flexure after 
480 days at all sites. This yellowing could only be observed in material N by viewing the edge of 
the test specimens. 

Microscopic Examination : Examination of the exposed surfaces of materials G, H, and J with optical 
and scanning electron microscopes revealed that the surface dulling observed visually, without magni- 
fication, was primarily due to the formation of microcracks (see figure 3.1.3). In general, the 
microcracking was more extensive at the Cape Canaveral and Palo Alto sites than at the other sites 
and had more open crack structure. In addition to the microcracking, the samples of material H 
exposed at the Palo Alto site and of material J at the Palo Alto and Cape Canaveral sites had evi- 
dence of surface etching. The microcracks observed for material J were not as open and extensive 
as those in materials G and H. 

Optical Measurements : Integrated solar transmittance values are given in table 3.1.2 for cover 
samples exposed on collector mini-boxes for up to 480 days at all four outdoor exposure sites. With 
the exception of material G at all of the outdoor sites, material H at Cape Canaveral, and material 
M at the Gaithersburg site, the changes measured for integrated transmittance were not substantial. 
Examination of the spectral transmittance curves for materials G, H, J, K, and N revealed the changes 
shown in figure 3.1.4. The hemispherical transmittance curves for material G at both the Cape 
Canaveral and Phoenix sites were virtually identical for up to 240 days. After 480 days of exposure, 
the transmittance curves dropped substantially with the greatest change occurring at the Cape Canaveral 
site. This most likely is related to the extend of microcracking occurring with these two materials. 
Examination of the curves obtained for material H using an integrating sphere showed no changes 
after 480 days at Phoenix or for up to 240 days at Cape Canaveral. After 480 days of exposure at 
Cape Canaveral, there was a drop in the transmittance curve for material H. Once again, this is 
most likely related to the size and extent of microcracking. There also was a slight shift in the 



* The term "absorption edge" as used in this report refers to the long wavelength side of the intense 
absorption band typically occurring in the 300 to 400 nm wavelength region in the spectral 
transmission curves shown in this report. 



18 



Table 3.1.1. Transm i ttance of Full-Size Collector Cover Material after Exposure. 



Site 
Exposure 



Collector Series Control Days** Phoenix Cape Canaveral Palo Alto Gaithersburg 



E 1 0.85 

2 
3 
it 

G* 1 0.84 

2 
3 
4 

H* 1 0.85 

2 



480 


0.78 


0.81 


0.81 


0.80 


480 


0.80 


0.78 


0.82 


0.81 


240 


0.83 


0.77 


-- 


-- 


60 


0.79 


0.82 


-- 


-- 


480 


0.71 


0.76 


0.77 


0.75 


480 


0.67 


0.54 


0.63 


0.80 


240 


0.60 


0.67 


-- 


-- 


60 


0.74 


0.66 


-- 


-- 


480 


0.85 


0.79 


0.80 


0.84 


480 


0.86 


0.81 


0.80 


0.84 



* Outer Cover 

** Days with a Minimum Solar Radiation Level of 17.000 kJ/sq m 



Table 3.1.2. Effect of 'Real Time' Outdoor Exposure on Cover Sample Transm i ttance 

Phoenix Cape Canaveral Palo Alto Gaithersburg 

Days Exposure* Days Exposure Days Exposure Days Exposure 

Sample Control 80 160 240 480 80 160 240 480 80 160 240 480 80 160 240 480 

0.83 0.83 0.80 0.84 0.84 0.81 0.81 0.83 0.85 0.82 0.80 0.77 0.82 0.81 0.80 0.81 

0.80 0.80 0.83 0.68 0.81 0.79 0.79 0.63 0.81 0.80 0.80 0.73 0.82 0.80 0.80 0.75 

0.85 0.85 0.85 0.85 0.84 0.85 0.85 0.79 0.84 0.85 0.83 0.84 0.84 0.86 0.87 0.85 

0.87 0.86 0.86 0.85 0.85 0.85 0.84 0.84 0.84 0.85 0.83 0.83 0.86 0.86 0.85 0.84 

0.89 0.90 0.90 0.90 0.90 0.90 0.84 0.90 0.90 0.89 0.91 0.89 0.90 0.90 0.90 0.90 

0.83 0.83 0.83 0.84 0.84 0.84 0.85 0.85 0.84 0.84 0.84 0.82 0.80 

0.80 0.74 0.78 0.78 0.78 0.77 0.73 0.75 0.80 0.81 0.74 0.80 0.74 0.75 0.76 0.70 

0.89 0.88 0.89 0.89 0.88 0.88 0.88 0.90 0.89 0.89 0.88 0.89 0.89 0.89 0.89 0.88 

0.89 -- -- 0.89 -- -- -- 0.89 — — — - - 

* 17000 kJ/sq-m (1500 Btu/sq-ft) Minimum Days 



19 



E 


0.85 


G 


0.84 


H 


0.85 


J 


0.88 


K 


0.89 


L 


0.84 


M 


0.78 


N 


0.90 





0.89 




300 500 700 900 1100 1300 1500 
WAVELENGTH (nm) 




u 

< 

£ 60 

E 

I 50 



< 

« 40 



52 30 - 



20 - 



10 



MATERIAL H 
(with integrating sphere) 



Control 

Phoenix 

Gaithersburg 

Cape Canaveral 

Palo Alto 



J I I L 



J I L 



300 600 700 900 1100 1300 1500 
WAVELENGTH (nm) 



Figure 3.1.2 Spectral transmittal curves for cover samples from full-size collectors after 
480 exposure days (^17,000 kJ/sq-m-day) 



20 





(a) Material H - Phoenix 



(b) Material H '- Cape Canaveral 





(c) Material H - Palo Alto 



(d) Material H - Gaithersburg 



Figure 3.1.3 Microcracking of cover samples after "real time" exposure on 
mini-boxes for 480 exposure days (>17,000 kJ[/sq-m-day) 



21 





(e) Material J - Phoenix 



(f) Material J - Cape Canaveral 





(g) Material J - Palo Alto 



(h) Material J - Gaithersburg 



Figure 3.1.3 continued 



22 




(i) Material G - Cape Canaveral 



Figure 3.1.3 continued 



23 





100 






MATERIAL G (a) 






(with integrating sphere) 




90 
80 










/ / 


^^^ ^\^\ /• 


|5 

UJ 

U 

z 


70 




/ 1 - 


v J v./"' 


< 

1- 


60 




' i /" 


*■ '' — .•' 


E 






/ / 




00 






/ / 




Z 






/ 


s^ . s "". 


ec 


50 


~~ (i ' ^ 


' ' s/ ~~' V / 


— i 

< 






Control 


o 

E 


40 


~ V' 


Phoenix & Cape Canaveral 


Ltl 

X 




'■■l 


(80. 160. & 240 days) 


M 


30 




;/ 


Accelerated aging (=2 yr) 


E 

UJ 




- 


J 


Phoenix (480 days) 




20 




Cape Canaveral (480 days) 




10 


J 


i i i 


1 1 I 1 1 I III 



300 



500 



700 900 1100 1300 1500 
WAVELENGTH (nm) 




MATERIAL H 
(with integrating sphere) 

Control 

Phoenix (80. 160, 240. 
& 480 days) 

Cape Canaveral (80. 160, 

& 240 days) 

Cape Canaveral (480 days) 

Accelerated aging (=2 yr) 



300 400 500 600 
WAVELENGTH (nm) 



700 



100 
90 
80 

E 70 

UJ 

U 

< 60 - 

i- 

E 

g 50 

< 
ec 
t- 
=! 40 h 

E 
ec 

1 30 
20 
10 - 

340 



- 




(c) 


- 


1 


MATERIAL H - PHOENIX 


— 


y 


(without integrating sphere) 
Control 


— 




80 days 

160 days 


- 1 




240 days 


- j 




480 days 


J 


i 


i i i 



100 
90 
80 
70 
60 
50 
40 
30 
20 
10 



(d) 



MATERIAL H - CAPE CANAVERAL 
(without integrating sphere) 




400 460 520 

WAVELENGTH (nm) 



580 



340 400 460 520 
WAVELENGTH (nm) 



580 



Figure 3.1.4 Spectral transmittance curves for cover samples exposed to "real time" and 
accelerated outdoor exposure on mini-boxes 



24 



100 
90 
80 

ft*-* 

B 70 

z 
< 

| 60 - 

V) 

z 
k 50 

_l 

< 

a 40 
u 

x 

| 30 

UJ 

x 
20 

10 



300 



(e) 




J_ 



MATERIAL J - PHOENIX 
(with integrating sphere) 

Control 

80 days 

160 days 

240 days 

480 days 

Accelerated aging 

(=2 yr) 



400 500 600 
WAVELENGTH (nm) 



700 



100 r- 
90 
80 



g 70 

UJ 

u 

< 60 

i- 

| 50 

< 
az 

*Z 40 

< 
E 

i 30 
20 
10 



(f) 




MATERIAL J - PHOENIX 
(without integrating sphere) 

Control 

80 days 

160 days 

240 days 

480 days 



340 400 460 520 

WAVELENGTH (nm) 



580 



640 



100 r 

90 

80 

H 70 

ui 

u 

< 60 

| 50 

< 

BE 

< 
E 
g 30 

z 

20 
10 



(9) 



MATERIAL J - CAPE CANAVERAL 
(without integrating sphere) 




-Control 



80 days 

160 days 

240 days 

480 days 



340 400 460 520 

WAVELENGTH (nm) 



580 



640 





100 




90 




80 




70 


g 




UI 




u 




z 
< 


60 


t 




E 

00 


50 


z 




< 




cc 




1- 


40 


< 




o 




BE 




UJ 

X 


30 


0. 
V) 






20 



10 



(h) 



—'7 



MATERIAL K 
(with integrating sphere) 

Accelerated aging (=2 yr) 



Control & Cape Canaveral 

(80. 160. 240. & 480 days) 
& Phoenix (80. 160. & 240 
days) 

Phoenix (480 days) 



300 400 500 600 

WAVELENGTH (nm) 



700 



Figure 3.1.4 Continued 



25 





100 




90 




80 






LU 
U 


70 


z 




< 




t- 




t- 


60 


z 




CO 




z 




< 


50 


h- 




_i 




< 




u 


40 



| 30 

LU 

z 
20 

10 



(i) 



MATERIAL N 
(with integrating sphere) 



— Control 

— Phoenix (80. 160. 240, 
& 480 days) & Cape 
Canaveral (80, 160, 240, 
& 480 days) & Accelerated 
aging (=2 yr) 



_L 



300 400 500 600 

WAVELENGTH (nm) 



700 



Figure 3.1.4 Continued 
26 



absorption edge towards shorter wavelengths which may be due to loss of UV inhibitor. Normal spectral 
transmittance measurements showed systematic changes in the transmittance of material H at both the 
Phoenix and Cape Canaveral sites with the greatest changes occurring at the Cape Canaveral site 
where the microcracking was more extensive. Hemispherical spectral transmittance curves measured 
for samples of material J exposed at Phoenix and Cape Canaveral showed identical shifts in the 
absorption edge to longer wavelengths as a function of exposure time (see figure 3.1.4e.). The 
similarity of these two curves is probably due to the fact that the microcracking in material J was 
not as open or extensive as that observed with materials G and H. Normal spectral transmittance 
curves showed greater changes for material J as a function of exposure time at Cape Canaveral than 
at Phoenix. With the exception of a slight shift in the absorption edge of material K to shorter 
wavelengths after 480 days of exposure at the Phoenix site, outdoor "real time" exposure of material 
K at the Phoenix and Cape Canaveral sites did not cause significant changes in the hemispherical 
transmittance. Measurements made without an integrating sphere on specimens from the two sites 
showed small decreases in transmittance as a function of exposure time and the same slight shift in 
the absorption edge towards shorter wavelengths for the 480 day Phoenix specimen. The significance 
of this shift in absorption edge will be discussed in section 3.1.3. A shift in the absorption 
edge of material N towards longer wavelengths was observed with spectral measurements made both with 
and without integrating spheres. In addition, a slight increase in scattering as a function of 
exposure time in Cape Canaveral was detected using the spectrophotometer without an integrating 
sphere. The shift in the absorption edge of material N to longer wavelengths is most likely due to 
photodegradation of the UV inhibitor as reported by Newland and Tamblyn [37], 

3.1.3 Accelerated Outdoor Testing 

Only the results obtained for cover samples mounted in mini-boxes will be discussed here because of 
peeling problems experienced with the absorber paint used in the board mount configuration. 

Visual Inspection : Accelerated outdoor exposure of material E for a period equivalent to 2 years 
"real time" gave rise to slight yellowing. Similar exposure of material G caused slight to moderate 
yellowing and evidence of exposed surface dulling. Material M had a slight increase in yellowing and 
a minor loss in surface gloss when exposed to these conditions. Accelerated outdoor exposure equiva- 
lent to 2 years "real time" caused noticeable top surface dulling and embrittlement of material H 
and a slight amount of yellowing and loss in gloss of the exposed surface in material J. Exposure 
under these conditions also caused a very slight amount of yellowing in material N which could only 
be observed by viewing the edge of the specimen and did not cause any visible changes in material K. 

Microscopic Examination : Examination of the exposed surfaces of materials G, H, and J revealed the 
presence of microcracking (see figure 3.1.5). The microcracks observed with accelerated outdoor 
aging generally appeared to be finer and less open than those observed with outdoor "real time" 
aging. 

Optical Measurements : Integrated solar transmittance values are given in table 3.1.3 for cover 
samples exposed outdoors on an accelerated weathering machine for a period of time equivalent to 2 
years "real time." Materials G, H, and M were the only materials which showed substantive changes in 
their transmittance in this time period. Examination of hemispherical spectral transmittance curves 
measured on materials G, H, J, K, and N revealed a number of changes resulting from this accelerated 
exposure (see figures 3.1.4a, b, e, h, and i) . The transmittance curve for material G was somewhat 
higher than those measured on "real time" samples exposed at Phoenix and Cape Canaveral for 480 days; 
however, it was lower than the curves of the 240 day samples. This is probably due to the less 
developed microcrack pattern that occurred on the samples of material G exposed to accelerated aging. 
The curve for material H was very similar to that obtained for the 480 day Cape Canaveral sample. 
Both of these samples had a well developed microcrack structure. The slight shift of the absorption 
edge towards shorter wavelengths that occurred with "real time" exposure of material H also occurred 
with the accelerated aging sample. As a result of the accelerated aging, the absorption edge of 
material K showed a large shift towards shorter wavelengths. This was probably due to the loss of UV 
inhibitor which permitted the material to transmit further into the ultraviolet. 

The same shift of the absorption edge of material N towards longer wavelengths observed with "real 
time" exposure also occurred with accelerated exposure. As was previously mentioned, this is 
probably due to photodegradation of the UV inhibitor into products which absorb at longer 
wavelengths . 



27 





(a) Material H - Sample 1 



(b) Material H - Sample 2 



«i9 



3X£3 





flf9S.'-' : ' 



wU Li Hi 




(c) Material G 



(d) Material J 



Figure 3.1.5 Microcracking of cover samples after accelerated exposure on 
mini-boxes equivalent to » 2 years "real time" 



28 



Table 3.1.3. Effect of Accelerated Outdoor Tests on Cover 
Sample Transmi ttance 







Sample l,Mi 


ni-box 


Sampl 


e 2, Mi 


ni-box 






Days 


Exposure* 


Days 


Exposure 


Sample Control 


30 


60 


120 


30 


60 


120 


E 


0.83 


0.83 


0.84 


0.63 


0.63 


0.83 


0.83 


G 


0.84 


0.83 


0.63 


0.78 


0.81 


0.82 


0.74 


H 


0.85 


0.8t4 


0.85 


0.81 


0.84 


0.85 


0.81 


J 


0.87 


0.86 


0.66 


0.84 


0.84 


0.66 


0.85 


K 


0.90 


0.93 


0.92 


0.92 


0.92 


0.92 


0.91 


L 


— 


— 


-- 


— 


— 


— 


— 


M 


0.82 


0.77 


0.79 


0.76 


0.81 


0.79 


0.80 


N 


0.91 


0.89 


0.89 


0.89 


0.89 


0.69 


0.89 



* Equivalent Days; 5 Equivalent Days = 1 Month Real Time 

= 6.625E+5 kj/sq m 



Table 3.1.4. Effect of Temperature Exposure on Cover Sample Transm i ttance 



Temperature: 




70 C 






90 C 






125 C 








Hours 


Exposi 


re 


Hours 


Exposure 


Hours 


Exposure 


Sample Control 


500 


1000 


2000 


500 1000 


2000 


500 1000 


2000 


E 


0.85 


0.83 


0.85 


0.81 


0.84 


0.84 


0.82 


0.82 


0.81 


0.79 


G 


0.84 


0.82 


0.80 


0.79 


0.73 


0.73 


0.78 


0. 70 


0.66 


0.64 


H 


0.85 


0.84 


0.85 


0.84 


0.84 


0.84 


0.85 


0.85 


0.85 


0.84 


J 


0.88 


0.87 


0.87 


0.87 


0.87 


0.87 


0.87 


0.87 


0.87 


0.86 


K 


0.89 


0.91 


0.91 


0.91 


0.90 


0.91 


0.91 


0.89 


0.87 


0.85 


L 


0.84 


0.83 


0.81 


0.83 


0.83 


0.80 


0.83 


0.81 


0.80 


0.80 


M 


0.78 


0.81 


0.79 


0.80 


0.75 


0.64 


0.66 


0.68 


0.63 


0.55 


N 


0.90 


0.89 


0.90 


0.90 


0.90 


0.90 


0.89 


0.89 


0.90 


0.91 



29 



3.1.4 Temperature Testing 

Visual Inspection: Laboratory exposure of materials E and L to temperature caused a slight amount of 
yellowing after 2000 h at 70°C and after 500, 1000, and 2000 h at 90°C. At 125°C, slight yellowing 
was observed after 500 h which became progressively deeper after 1000 h and 2000 h of exposure. 
Exposure to a temperature of 70°C caused very slight yellowing of material G after 1000 h which 
became slightly deeper at 2000 h. A moderate amount of yellowing occurred in this material at 90°C 
after 500, 1000, and 2000 h of exposure. At 125°C, this material had very severe yellowing after 
500 h which became progressively worse at 1000 and 2000 h. Laboratory exposure of material M to a 
temperature of 70°C caused a slight increase in yellowing after 500 h that increased slightly after 
2000 h of exposure. A slight to moderate amount of yellowing occurred at 90° C after 500 h, which 
deepened to moderate at 2000 h. At 125°C, material M had severe yellowing after 500 h which became 
progressively worse at 1000 h and 2000 h. The top surface also exhibited a wrinkled pattern. 
Laboratory exposure of material H to temperatures of 70°C, 90°C, and 125°C did not cause any visible 
changes with the exception of possible very slight yellowing after 2000 h at 125°C. Material J 
showed no visible changes after exposure for up to 2000 h at 70°C and 90°C and 125°C. Laboratory 
exposure of material K to temperatures of 70°C and 90°C did not cause any visible changes in 2000 h. 
At 125°C, there was moderate yellowing after 500 h which became progressively more severe with expo- 
sures up to 2000 h; however, the film still remained flexible. Similarly, laboratory exposure of 
material N for up to 2000 h at 70°C and 90°C did not cause any visible changes. After 1000 h at 
125°C, there was noticeable sagging and warping and a minor amount of yellowing that could only be 
seen by looking at the edge of the specimen. 

Microscopic Examination : Examination of the surface of a specimen of material H, heated for 
2000 h at 125°C, with a scanning electron microscope revealed a considerable number of small surface 
blisters (see figure 3.1.6). Changes were not observed for any of the other cover materials exposed 
to temperature in this program. 

Optical Measurements : Integrated solar transmittance values are given in table 3.1.4 for cover 
samples exposed for up to 2000 h at temperatures of 70°C, 90°C, and 125°C. Material G was the only 
material to show a change at 70°C. At 90°C, materials G and M had substantial changes. At 125°C, 
materials G and M had greater changes than those observed at lower temperatures and materials E, K, 
and L showed slight changes. Spectral transmittance curves are shown for materials G, H, J, and K as 
a function of exposure temperature and exposure time in figure 3.1.7. The absorption edge of 
material G, measured on a spectrophotometer with an integrating sphere, shifts to longer wavelengths 
as temperature and exposure time increase. This accounts for the changes in yellowing observed 
visually. The absorption edge of material H had a substantial shift to shorter wavelengths at 125°C. 
As was previously mentioned, this shift is most likely due to loss of UV inhibitor. Material J 
shows a slight rounding off of the top of the absorption edge as a function of exposure temperature 
and time. This is probably indicative of the onset of degradation. Measurements made on material K 
using an integrating sphere show an initial shift in the absorption edge to shorter wavelengths 
followed by a gradual shift back towards longer wavelengths. Measurements made on material K on a 
spectrophotometer without an integrating sphere show that this shift back towards longer wavelengths, 
following the loss of the UV inhibitor at temperatures as low as 70°C may be at least partially due 
to either an increase in scattering or film shrinkage. The spectral curves measured for material N 
using an integrating sphere did not show any signs of change after 2000 h at 125°C. 

3.1.5 Temperature and Humidity Testing 

Visual Inspection : Exposure to combined moisture and temperature caused moderate whitening of 
materials E and L at 70°C and a combination of severe whitening, blister formation, and yellowing at 
90°C which became progressively worse with exposure time (500 h to 2000 h) . Exposure of material G 
to combined moisture and temperature caused severe whitening after 500 h at 70°C which became more 
severe at longer times. At 90°C with moisture, there was a combination of very severe whitening and 
yellowing which progressed from moderate to very severe as the exposure time increased from 500 h to 
2000 h. The surfaces remained glossy but changed in appearance from the unexposed specimens. Expo- 
sure of material M to combined moisture and temperature caused severe whitening after 500 h at 70°C 
and evidence of large blisters on the bottom surface. The top surface became very sticky after this 
exposure. At 90°C with moisture, there was a combination of severe whitening and yellowing which 
progressed from moderate after 500 h to a deep tan after 2000 h. There was also a noticeable change 
in top surface texture and evidence of large blisters on the bottom surface. Exposure of material H 
to combined moisture and temperature caused a slight amount of yellowing after 1000 at 70°C; however, 
the material remained flexible. At 90°C with moisture, there was slight yellowing and material H 
became brittle after 500 h of exposure; after 1000 h, there was moderate yellowing and some clouding 
of the material in addition to the embrittlement and after 2000 h the material broke into small 
pieces. Exposure of material J to combined moisture and temperature caused no visual changes at 

30 




Figure 3.1.6 Blistering of cover material H after 2000 h at 125°C 



31 



100 


— 


(a) 


100 




(b) 


90 


- 


90 




80 

5? 


- 




80 




g 70 

z 
< 

t 60 




§ 1 

'{'■ /j MATERIAL G 

\j I j"jj (with integrating sphere) 

[/ /// Control 

[1 // 70"C-500h 

j !// 70°C-1000h 


B 70 

z 
< 

| 60 

c/s 

z 

2 50 

< 




i 

i 

MATERIAL H 


E 

CO 

z 

2 50 

►- 
_■ 
< 

| 40 


_ 


.// 
ii, 

-in 


(with integrating sphere) 

Control 

70°C-2000h 

90"C-2000h 


LU 

x 




| U\ 70°C-2000 h 


UJ 

X 


i/i 


125T-1000 h & 2000 h 


1 30 




/ /'/' 90"C-500 h 


a. 
£ 30 


J/ i 


125°C-500h 


E 

UJ 

X 

20 


— 


j ilj 125°C-500 h 

/< 125°C-1000 h 

\j< 125°C-2000h 

H] 


S 

UJ 

X 

20 


! 

f ! 

1 i 




10 




10 


1 i 
j 






— ! 


tr i i i i i i i i i i i 




fcft/l 1 1 1 1 1 1 1 1 1 1 1 



300 500 700 900 1100 
WAVELENGTH (nm) 



1300 1500 



300 500 700 900 1100 
WAVELENGTH (nm) 



1300 1500 



100 
90 

80 

E 

uj 70 

z 
< 

I 60 

V) 

z 
S 50 

< 

£ 40 



I 30 



20 
10 



LJ 



(C) 



100 




MATERIAL J 
(with integrating sphere) 

Control 

125°C-2000h 

- 70°C-500. 1000, & 

2000 h & 90°C-500. 
1000. & 2000 h& 
125°C-500& 1000 h 



J I I L 



J I L 



300 500 700 900 1100 
WAVELENGTH (nm) 



1300 



u 

z 
< 




50 



£ 40 



Q. 

£ 30 



20 



10 i 



: I 



J 



MATERIAL K 
(with integrating sphere) 

Control 

70°C-2000h& 

90-C-2000 h 

125"C-500h 

125'C-IOOOh 

125°C-2000h 



_L 



300 500 700 900 1100 
WAVELENGTH (nm) 



1300 



Figure 3.1.7 Spectral transmittance curves of cover samples after temperature aging 

32 





80 




70 




60 


UJ 




U 




z 




< 
1- 


50 


h- 




Z 




V) 

z 


40 


< 




ce 




t— 




_i 


30 


< 




£ 




cc 




o 


211 


z 






10 



MATERIAL K 
(without integrating sphere) 



(e) 




Control 
70°C-500 h 
70X-1000 h 
70°C-2000 h 



200 300 400 500 

WAVELENGTH (nm) 



§00 



700 



Figure 3.1.7 Continue^ 
33 



70°C. At 90°C with moisture, the material started to whiten after 1000 h, and was considerably 
whiter after 2000 h. The material also became progressively more brittle as a result of this 
exposure; however, its surfaces remained glossy. Exposure of material K to moisture at temperatures 
of 70°C and 90°C for up to 2000 h caused no visible changes. Material N showed no visible signs of 
change after exposure to moisture at 70°C. After 500 h of exposure to moisture at 90°C, the material 
started to take on a cloudy white appearance, which became progressively more severe at longer expo- 
sure times. The cloudy white appearance was accompanied by a noticeable increase in embrittlement 
when specimens were broken in flexure; however, the specimen surfaces remained glossy indicating 
that surface microcracking was probably not the cause of this whitening. 

Microscopic Examination : Examination of materials E, G, L, and M with an optical microscope. revealed 
that the whitening of these materials, when exposed to temperature in combination with moisture, was 
probably due to delamination between the glass fibers and the resin in the bulk of the material. 
This type of resin-glass fiber separation was not observed with specimens exposed outdoors to either 
"real time" or accelerated conditions or in any other type of laboratory exposure used. Surface 
microcracking was observed in specimens of material G after 500 h of exposure to moisture at 90°C; 
but not at 70°C with moisture. The change in surface texture observed visually for material M at 
90°C with moisture was also found to be due to the formation of surface microcracks. Examination 
with a scanning electron microscope of specimens of material H, exposed to moisture at 90°C, indicated 
a considerable amount of moisture etching; but no microcracking (see figure 3.1.8). Similar moisture 
etching was also observed on the Palo Alto "real time" exposure test specimen after 480 days. Exami- 
nation of the surfaces of materials J, K, and N after exposure to combined moisture and temperature 
showed no noticeable changes. 

Optical Measurements : Integrated solar transmittance values are given in table 3.1.5 for cover 
samples exposed for up to 2000 h at temperatures of 70°C and 90°C with a relative humidity of approx- 
imately 95 percent. With the exception of the sample of material H which disintegrated after 2000 h 
at 90°C and could not be measured, all of the materials showed very large changes in their transmit- 
tance at this temperature. After 1000 h at 70°C, materials H, K, and N did not show changes in 
integrated transmittance. The remainder of the materials showed substantial changes which were 
somewhat less than those which occurred at 90°C with humidity. Spectral transmittance curves are 
shown for materials G, H, J, K, and N in figure 3.1.9. Material G showed a drastic decrease in 
spectral transmittance as a result of exposure to temperature and humidity. The other glass fiber 
reinforced plastics performed somewhat better, but also showed substantial decreases in performance. 
Measurements of material H using an integrating sphere showed a slight shift in the absorption edge 
to shorter wavelengths at 90°C. The curves measured for this material without an integrating sphere, 
which are shown in figure 3.1.9b, show a substantial decrease in transmittance, probably due to scat- 
tering, after 1000 h of exposure to humidity at 90°C; the 2000 h samples broke into small flakes. 
Materials J and N showed similar changes in their spectra, both with and without an integrating 
sphere. These changes, which were much greater without an integrating sphere, are most likely due to 
scattering which would account for the cloudy white appearance observed visually for these materials. 
Material K showed the same loss in UV inhibitor observed for exposure to temperature without humidity 
with a shift of the absorption edge towards shorter wavelengths. 

3.1.6 Temperature and Xenon Arc Radiation Testing 

Visual Inspection : Temperature and xenon arc radiation exposure for up to 2000 h at 70°C caused no 
visual signs of degradation in materials E and L. After 500 h at 90°C with xenon arc radiation, a 
slight amount of yellowing was observed in these materials which became progressively more severe 
with exposure time. Similarly, material G showed no visible signs of degradation with xenon arc 
radiation at 70°C; however, at 90°C there was severe yellowing after 500 h of radiation exposure 
which became progressively more severe with exposure time. Temperature and xenon arc exposure of 
material M for 500 h at 70°C caused a slight increase in yellowing after 500 h which became slightly 
more intense with exposure time. At 90°C with xenon arc radiation, there was slight yellowing after 
500 h which progressed to severe yellowing after 2000 h. No visible signs of degradation occurred 
in material H after exposure to temperature and xenon arc radiation for 2000 h at 70°C and after 
1000 h at 90°C; after 2000 h of exposure at 90°C, there was a slight amount of yellowing and the 
material became quite brittle. In the case of material J, temperature and xenon arc radiation caused 
a moderate amount of yellowing after 500 h at both 70°C and 90°C. This yellowing appeared to remain 
fairly constant for exposure times up to 2000 h. Material K showed no visible signs of degradation 
after radiation exposure for 2000 h at 70°C and 500 h at 90°C. After 1000 h at 90°C, there was a 
slight amount of yellowing and after 2000 h of xenon arc radiation exposure at 90°C, material K 
became moderately yellow and brittle. Exposure of material N to temperature and xenon arc radiation 
caused no visible signs of degradation after 2000 h at 70°C. After 500 h of radiation exposure at 
90°C, a slight amount of yellowing could be seen on viewing the edge of the test specimen. Specimens 



34 



(a) 500 hours 




(b) 1000 hours 




(c) 2000 hours 




Figure 3.1.8 Surface changes in cover material H after 500, 1000, and 
2000 h at 90°C and 95% RH 



35 



Table 3.1.5. Effect of Temperature & Humidity on Cover 

Sample 7 ransm i ttance 



70 C and 95% RH 90 C and 95% RH 
Hours Exposure Hours Exposure 
Sample Control 500 1000 2000 500 1000 2000 



E 


0.85 


0.72 


0.64 


C 


0.814 


0.15 


0.13 


H 


0.85 


0.85 


0.85 


J 


0.88 


0.87 


0.87 


K 


0.89 


0.90 


0.91 


L 


0.84 


0.54 


0.51 


M 


0.78 


0.35 


0.31 


N 


0.90 


0.88 


0.89 



0.50 


0.46 


0.30 


0.10 


0.08 


0.07 


0.84 


0.83 


— 


0.86 


0.84 


0.74 


0.91 


0.91 


0.87 


0.31 


0.25 


0.21 


0.24 


0.23 


0.18 


0.76 


0.82 


0.64 









Table 3.1.6. Effect of Temperature & Xenon Arc Radiation 
on Cover Sample T ransm i ttance 







70 C and Xenon Arc 


90 C and Xenon Arc 






Hours 


of Exp 


osure 


Hours 


of Exposure 


Samp I e 


Contro I 


500 


1000 


2000 


500 


1000 


2000 


E 


0.85 


0.85 


0.85' 


0.84 


0.78 


0.79 


0.77 


G 


0.84 


0.80 


0.78 


0.81 


0.78 


0.78 


0.74 


H 


0.85 


0.85 


0.85 


0.85 


0.86 


0.86 


0.85 


J 


0.88 


0.86 


0.86 


0.86 


0.85 


0.85 


0.84 


K 


0.89 


0.92 


0.93 


0.92 


0.91 


0.89 


-- 


L 


0.84 


0.82 


0.84 


0.83 


0.83 


0.83 


0.80 


M 


0.78 


0.79 


0.78 


0.80 


0.74 


0.73 


0.70 


N 


0.90 


0.89 


0.89 


0.90 


0.89 


0.88 


0.88 



36 



100 
90 
80 
70 



t 60 

E 

CO 



50 



< 
H 40 



ffi 30 



20 
10 



(a) 




(with integrating sphere) 

Control 

70°C & 95%RH-500 h 

70°C&95% RH-IOOOh 

90°C & 95% RH-500 h 

90°C& 95% RH-IOOOh 

90°C & 95% RH-2000 h 



1 



£t 



-:\ /. -x 



" Sfl«^%/ 



/:- 



*& 



J L 



I I I I 



300 500 700 900 1100 
WAVELENGTH (nm) 



1300 1500 



CJ 

z 
< 



< 
ee 



90 




(b) 


80 




s^^^^^ 


70 


// 
// 




60 


// 
// 
/ 

/ 
/ 
/ 




50 


.... 


40 


' ''' 
' /■'' 


MATERIAL H 
{without integrating sphere) 


30 


/ /■' 


Control 




/ ■' 
/ f 


90"C & 95% RH-500 h 


20 
10 


/ / 
/ 
/ 1 

/ 7 


90°C& 95% RH-IOOOh 

i i i i 



340 



400 



460 520 
WAVELENGTH (nm) 



580 



640 



100 





90 




80 






LU 

o 


70 


z 




< 




(- 




1— 


60 


£ 




CO 




&. 




< 


50 


1- 




_ 1 




< 




u 


40 


ec 




UJ 




z 




0. 




CO 


30 


1=2 




LU 




X 






20 




10 



(c) 




MATERIAL J 
(with integrating sphere) 

Control 

70°C&95%F? 

90°C & 95% RH-500 h 

90°1&95%RH-1000 h 

90°C& 95% RH-2000 h 



J L 



J L 



J L 



J L 



300 500 700 900 1100 
WAVELENGTH (nm) 



1300 



100 

90 

80 

B 70 

LU 

u 

< 60 



~ ,„ 



^ 



40 
30 
20 
10 



(d) 



MATERIAL J 
(without integrating sphere) 




f/^-90"- 




400 460 520 580 640 
WAVELENGTH (nm) 



Figure 3.1.9 Spectral transmittance curves of cover samples after temperature and humidity aging 

37 



100 
90 
80 
70 
60 
50 
40 
30 
20 
10 h 




MATERIAL N 
(with integrating sphere) 

Control 



70°C & 95% RH-500 & 1000 h 

90"C & 95% RH-500 h 

90°C&95%RH-1000h 

90°C&95%RH-2000h 



J_ 



300 500 700 900 1100 
WAVELENGTH (nm) 



1300 



u 

z 
< 



E 
in 



< 
E 
cc 
o 



100 
90 
80 
70 
60 
50 
40 
30 
20 
10 



(f) 



MATERIAL N 
(without integrating sphere) 




Control 

90°C& 95% RH-500 h 

90°C&95%RH-1000h 

90°C&95%RH-2000h 



/ 



/ 



/" 



K^ 



300 400 500 600 

WAVELENGTH (nm) 



700 



100 


- 






(g) 


90 
80 


4 


s~' 


. -^..^/ 


J? 

£ 70 

as 

t 60 

E 

V) 

as 


i 

'! 






MATERIAL K 
(with integrating sphere) 

Control 


g 50 


"I 






70°C & 95% RH-500 h 


_l 

< 


/ 
i 






70°C&95%RH-1000h 


a 40 

ce 


1 






90°C & 95% RH-500 & 1000 h 


HEMISPHE 
w 


1 






90°C & 95% RH-2000 h 


20 


— 








10 


\ 


i 


i 


i i i i i i i i 



300 



500 



700 900 1100 
WAVELENGTH (nm) 



1300 





80 




70 


H 


60 


UJ 




U 




z 




< 
1— 


50 


1- 




S 




z 


40 


«t 




cc 




i— 






30 


< 




E 




cc 




o 


XII 


z 






10 



(h) 



MATERIAL K 
(without integrating sphere) 

Control 

90"C & 95% RH-2000 h 

90'C&95%RH-1000h 
90°C& 95% RH-500 h 




220 



280 



340 400 

WAVELENGTH (nm) 



460 



520 



580 



Figure 3.1.9 Continued 



38 



exposed for 1000 h and 2000 h under these conditions became noticeably more brittle on breaking in 
flexure in addition to showing this slight yellowing. 

Microscopic Examination: Surface raicrocracking was observed in the specimen of material J exposed to 
xenon arc radiation at 90°C for 2000 h. No other changes were observed for test specimens exposed 
to xenon arc radiation in conjunction with temperature. 

Optical Measurements : Integrated solar transmittance values are given in table 3.1.6 for cover 
samples exposed to xenon arc radiation for up to 2000 h at temperatures of 70°C and 90°C. Material G 
was the only material which showed a change at 70° C. At 90° C, materials E, G, J, L, and M showed 
significant changes in their integrated transmittance. Examination of spectral curves obtained for 
material G showed shifts in the absorption edge to longer wavelengths as a function of exposure time 
and temperature (see figure 3.1.10a). This shift probably accounts for the yellowing observed 
visually for this material. Material H showed a slight shift in its absorption edge towards shorter 
wavelengths similar to that observed with the exposure tests discussed in preceding sections of this 
report. 

Changes in the absorption edge of material K, similar to those previously found for exposure to 
temperature and temperature and humidity, were also found after exposure to temperature and xenon arc 
radiation. Material N exhibited the same spectral shift in its absorption edge observed previously 
in outdoor exposure (see figure 3.1.41). Material J showed a slight shift in its absorption edge 
which increased with the exposure time and temperature (see figure 3.1.10b). This probably accounts 
for the yellowing observed visually for this material. 

3.1.7 Comparison and Assessment of Test Procedures 

The "real time" outdoor mini-box testing and accelerated outdoor testing both appear to be good ways 
of duplicating the types of changes observed with full-size collectors. However, outdoor "real 
time" exposure of 480 days having a minimum solar radiation level of 17,000 kJ/m^/day is required with 
the cover mini-boxes to make many of the changes observed at shorter times with full-size stagnating 
collectors evident. The accelerated outdoor test appears to be capable of doing this in 120 actual 
exposure days. This is most likely because the temperatures of cover samples mounted on the mini- 
boxes were typical of those measured on operating solar collectors, whereas those of samples in 
the accelerated test apparatus were typical of those measured on the covers of stagnating collectors. 
The changes used for comparative purposes included spectral changes, microcracking, embrittlement 
on bending, visual yellowing, and in the case of PMMA, molecular weight measurements (see 
table 3.1.7). The amount and extent of microcracking observed in outdoor "real time" exposure 
appeared to be closely related to the amount of moisture and condensation present at the exposure 
site with the greatest changes observed at sites with high prevailing humidity, i.e., Cape 
Canaveral rather than Phoenix. The cumulative amounts of solar radiation received at all four sites 
were roughly comparable (see Appendix A). The microcrack patterns observed with accelerated outdoor 
testing appeared to be finer and less open than those observed with outdoor "real time" exposure. This 
is probably due to the higher stress levels and loading rates caused by exposure to concentrated 
solar radiation in conjunction with an intermittent water spray. 

The indoor laboratory tests were able to duplicate some but not all of the changes observed outdoors. 
The temperature and xenon arc radiation tests appeared to be reasonable methods for determining 
changes due to these parameters. However, the indoor temperature and humidity testing produced 
changes that were not observed in outdoor exposure under conditions representative of those occur- 
ring in actual solar collectors. The primary value of this type of long-term test would be for 
glazings for trickle down collectors and polymeric water storage tanks where continuous exposure to 
moisture at elevated temperatures is likely. None of the indoor laboratory tests duplicated the 
extensive microcracking observed outdoors. Some of the glass fiber reinforced plastics produced 
microcracking as a result of temperature and moisture exposure; however, as mentioned above, the 
exposure conditions used in this study for humidity testing are believed to be too severe, i.e., 
there was extensive delamination between the glass fiber reinforcement and the resin in all of the 
glass fiber reinforced plastics subjected to this test. 

With regard to optical property measurements, emphasis in current ASTM methods concerned with the 
durability of cover materials [8, 9, 11] has been placed on the use of integrated spectral transmit- 
tance values. These integrated values are not sensitive to spectral changes that occur in a limited 
part of the solar spectrum, i.e., at short wavelengths in many polymers. Since little or no 
energy is found in the solar spectrum in this short wavelength region, integrated solar spectral 
transmittance values are of little value in detecting these changes which are sensitive indicators of 
degradation in many polymers. More emphasis should be placed on the analysis of normal and 
hemispherical spectral transmittance curves. 

39 



100 
90 

80 
70 
60 
50 
40 
30 
20 
10 



(a) 




MATERIAL G 
(with integrating sphere) 

Control 



70°C & Xenon arc - 500 h 

70°C & Xenon arc - 1000 h 

70°C & Xenon arc - 2000 h 

90°C & Xenon arc - 1000 h 

S0°C & Xenon arc - 2000 h 



j_ 



J_ 



_L 



300 500 700 900 1100 
WAVELENGTH (nm) 



1300 1500 



u 

z 
< 



z 



100 






(M 


90 

to 




1 


V w 


70 


- 


II 
f 


MATERIAL J 


60 




/■J 

n 
i 
i 
j 


(with integrating sphere) 
Control 






70°C & Xenon arc - 500, 1000. 


50 




& 2000 h 






90°C & Xenon arc - 500 h 


40 


— 




90°C & Xenon arc - 1000 h 

90"C & Xenon arc - 2000 h 


30 


- 






20 


- 






10 


"i 


i i 


I I i I I I I l l I 



300 



500 



700 900 1100 
WAVELENGTH (nm) 



1300 1500 



Figure 3.1.10 Spectral transmit tance curves of cover samples after temperature and xenon 
arc radiation aging 



40 



m oo n m — r*. m en en 
»* en co n mm en en en 



m en qo -a cm 

(SI cm — t *tf *^- 



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-j- co m 



m m m 
Ed W EO 


CO 


m m 

co to 


Ed 


in m 

CO CiJ 


Ed 


in m 

CO CO 


in in 

to to 1 


in in m 

to to to 


oo — < r-. 


r^ 


o oo 


m 


*£> m 


~i 


f-*. <■ 


CM O 1 


noNtn 


r* rv m 


VJD 


v£> »J 


v© 


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CO 00 


00 r*v r*» 



o o o o o o 

o o o o o o 

moo m o o 

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moo 

— * r»l 



o o o 
o o o 
moo 

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o o o o o o 

o o o o o o 

moo moo 

— < es — < CM 



55 



vo < n n 



co co en cm 






"■3" co en en 



in in 
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m m m m 
to to to CO 


in 

CO 


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to co to 


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m 
Ed 


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3.2 ABSORBER MATERIALS TESTING 

In this section, the results of outdoor and indoor laboratory exposure testing of absorber materials 
are presented and the extent to which accelerated outdoor and indoor laboratory tests simulate "real 
time" outdoor exposure tests are discussed. A complete listing of materials used for these compari- 
sons and their code letters are presented in table 2.2.4 of this publication. These materials were 
selected as being representative of the broad variety of absorber materials being used in flat-plate 
solar collectors when this test program was initiated. In addition to the small-scale tests which 
were performed on all materials, samples of those absorber materials having code letters A through H 
were cut from full-size solar collectors and evaluated following the completion of collector exposure 
testing as described in section 2 of this publication. The types of small-scale outdoor and indoor 
laboratory tests performed on absorber materials are also summarized in section 2. 

3.2.1 Full-Size Collector Stagnation Testing 

Visual Inspection : Whitish areas appeared on the absorptive coatings of several of the type A 
collectors within the first few months of outdoor exposure (see figure 3.2.1). There were no 
obvious environmental trends with regard to the occurrence of this whitening, i.e., one of the two 
type A collectors exposed at the Palo Alto site had extensive whitening and the other showed no 
evidence of this phenomenon. The source of this whitening has been identified by Moore as a crystal- 
line zinc salt [38]. Large dark gray irregular areas appeared on the absorptive coatings of most 
of the type C collectors exposed outdoors. These collectors had considerable evidence of water 
leakage and condensation which may have been the source of the problem. Pitting and, in some cases, 
extensive corrosion occurred on the absorbers of most of the type D collectors (see figure 3.2.2). 
Once again, there was no consistent trend that could be observed with regard to outdoor exposure 
site location and the degree of pitting and corrosion may have been caused by water leakage which 
could vary with the construction and assembly of individual collectors. The absorbers of collector 
types E, F, G, and H showed no visual signs of deterioration. 

Optical Measurements : Tables 3.2.1 and 3.2.2 present values for absorptance and emittance, 
respectively, measured on samples taken from the absorber plates of full-size collectors A through H 
following the completion of outdoor exposure and thermal performance testing. The nonselective 
coatings did not show any significant changes in optical performance as a result of exposure for 480 
days. The absorptance of coating A changed from an original value of 0.87 to as low as 0.79 and the 
emittance from 0.13 to as high as 0.30 for the series 1 and series 2 collectors. These changes in 
emittance appeared to be associated with the whitish deposits observed visually, with the largest 
change occurring in the series 2 collector exposed in Palo Alto. The series 4, Type A, collector 
exposed to stagnation conditions with radiation augmentation reflectors in Phoenix had a larger 
change in emittance to 0.43; however, the whitish deposits on this sample did not visually appear to 
be as extensive as those on the Palo Alto Series 2 collector. Changes in emittance for coating A 
were minimal at the Gaithersburg site, which correlated with the lack of visual signs of change for 
samples exposed there. Moore [38] measured values in emittance of 0.18 in the whitish areas of 
absorber samples taken from type A collectors used in an operational system for 2 years. As observed 
by Moore, variations in the color of absorptive coating A from tan to bluish-purple did not affect 
the optical properties of this material. Such colors were observed on both exposed and unexposed 
materials in this program. Increases in the emittance of absorber material D from 0.06 to as much 
as 0.17 are related to the amount of corrosion that occurred. Significant changes in absorptance 
were not observed for this material. The emittance of material F improved from 0.75 to as low as 
0.50. Accompanying this improvement in emittance was a decrease in absorptance from 0.96 to as low 
as 0.88. Samples of absorber material P, which were taken from collectors used in a solar energy 
system in Gaithersburg, Maryland, showed no significant changes in optical properties. These col- 
lectors had been in use for 5 years with a little more than a year of that time consisting of 
stagnation exposure. 

3.2.2 "Real Time" Coupon Specimen Testing 

Visual Inspection : The only materials which showed visual evidence of changes were material N, which 
changed in color from dark brown to tan within 80 days of exposure at all four sites, and material H, 
which showed large gray areas lighter than the original coating after 480 days of exposure at the 
Palo Alto site. All of the paints used as absorptive coatings gave off condensible outgassing 
products. This resulted in the appearance of cloudy areas on the glazing of the absorber exposure 
box directly above the paint coupon specimens. There were no visible signs of the whitening and 
corrosion observed for full-size collectors. 

Optical Measurements : Tables 3.2.3 and 3.2.4 present values for absorptance and emittance, 
respectively, measured as a function of exposure time for coupon specimens exposed on the selective 

42 




Figure 3.2.1 Whitish deposits on absorber material A sample from a full-size collector 
after 480 exposure days (^17,000 kJ/sq-m-day) 




Figure 3.2.2 Corrosion of absorber material D sample from a full-size collector after 
480 exposure days (^-17,000 kJ/sq-m-day) 



43 



Table 3.2.1. Absorptance of Full-Size Collector Absorber Coatings after Exposure, 















S i te 






Series 


Contro 1 


Exposure 
Days** 










Co 1 1 ector 


Phoen ix 


Cape Canavera 


1 Palo Alto Gaithersburg 


A 


1 


0.87 


480 


0.79 


0.83 


0.86 


0.84 




2 


0.87 


480 


0.83 


0.82 


0.82 


0.84 




3 


0.87 


240 


0.86 


0.84 


-- 


-- 




4 


0.87 


60 


0.88 


0.83 


-- 


-- 


B 


1 


0.98 


480 


0.98 


0.97 


0.97 


0.97 




2 


0.98 


480 


0.98 


0.97 


0.97 


0.97 




3 


0.98 


240 


0.97 


0.96 


-- 


-- 




4 


0.98 


60 


0.98 


0.97 


-- 


-- 


C 


1 


0.98 


480 


0.96 


0.96 


0.95 


0.97 




2 


0.98 


480 


0.96 


0.96 


0.96 


0.97 




3 


0.98 


240 


0.96 


0.96 


-- 


-- 




4 


0.98 


60 


0.97 


0.97 


-- 


-- 


D 


1 


0.97 


480 


0.95 


0.95 


0.95 


0.95 




2 


0.97 


480 


0.95 


0.95 


0.96 


0.96 




3 


0.97 


240 


0.95 


-- 


-- 


-- 




4 


0.97 


60 


-- 


-- 


-- 


-- 


E 


1 


0.95 


480 


0.96 


0.95 


0.96 


0.96 




2 


0.95 


480 


0.96 


0.95 


0.96 


0.95 




3 


0.95 


240 


0.95 


0.94 


-- 


-- 




4 


0.95 


60 


0.96 


0.95 


-- 


-- 


F 


1 


0.96 


480 


0.91 


0.90 


0.90 


0.88 




2 


0.96 


480 


0.90 


0.91 


0.89 


0.89 




3 


0.96 


240 


0.94 


0.93 


-- 


-- 




4 


0.96 


60 


0.91 


0.90 


-- 


-- 


H 


1 


0.95 


480 


0.95 


0.95 


0.95 


0.95 




2 


0.95 


480 


0.95 


0.95 


0.95 


0.95 




3 


0.95 


240 


0.95 


0.95 


-- 


-- 




4 


0.95 


60 


0.95 


0.95 


-- 


-- 


PI 


- 


0.96 


* 








0.93 


P2 


~ 


0.96 


■X- 








0.96 



* Samples from NBS Townhouse Collectors after Approximately 5 Years Exposure. 
** Days with a Minimum Solar Radiation level of 17,000 kj/sq m 



44 



Table 3.2.2 Emittance of Full-Sized Collector Absorber Coatings after Exposure. 















S i te 






Ser i es 


Cont ro 1 


Exposure 
Days** 










Co 1 lector 


Phoen i x 


Cape Canavera 


I Palo Alto Gaithersburg 


A 


1 


0. 13 


480 


0. 12 


0. 15 


0. 18 


0. 14 




2 


0.13 


480 


0. 17 


0. 13 


0.31 


0. 12 




3 


0. 13 


240 


0. 12 


0. 13 


-- 


-- 




4 


0. 13 


60 


0.43 


0. 13 


-- 


-- 


B 


1 


0.91 


480 


0.91 


0.91 


0.91 


0.91 




2 


0.91 


480 


0.91 


0.91 


0.91 


0.91 




3 


0.91 


240 


0.91 


0.91 


-- 


-- 




4 


0.91 


60 


0.91 


0.91 


-- 


-- 


C 


1 


0.92 


480 


0.90 


0.91 


. 90 


0.90 




2 


0.92 


480 


0.90 


0.91 


0.91 


0.91 




3 


0.92 


240 


0.91 


0.91 


-- 


-- 




4 


0.92 


60 


0.91 


0.90 


-- 


-- 


D 


1 


0.07 


480 


0.06 


0.06 


0.06 


0.14 




2 


0.07 


480 


0.06 


0. 10 


. 06 


0.06 




3 


0.07 


240 


0.06 


— 


-- 


-- 




4 


0.07 


60 


-- 


-- 


-- 


-- 


E 


1 


0.87 


480 


0.88 


0.87 


0.88 


0.87 




2 


0.87 


480 


0.89 


0.88 


0.88 


0.87 




3 


0.87 


240 


0.86 


0.86 


-- 


-- 




4 


0.87 


60 


0.87 


0.87 


-- 


-- 


F 


1 


0.75 


480 


0.56 


0.55 


0.50 


0.53 




2 


0.75 


480 


0.60 


0.58 


0.57 


0.53 




3 


0.75 


240 


0.63 


0.54 


-- 


-- 




4 


0.75 


60 


0.55 


0.50 


-- 


-- 


H 


1 


0.89 


480 


0.89 


0.88 


0.89 


0.88 




2 


0.89 


480 


0.89 


0.89 


0.89 


0.88 




3 


0.89 


240 


0.88 


0.89 


-- 


-- 




4 


0.89 


60 


0.89 


0.88 


-- 


-- 


P1 


- 


0.08 


•* 








0.07 


P2 


- 


0.08 


w 








0.08 



* Samples from NBS Townhouse Collectors after Approximately 5 Years Exposure, 
** Days with a Minimum Solar Radiation Level of 17,000 kJ/sq m. 



45 



Table 3.2.3. Effect of Outdoor Exposure on Absorber Coating Absorptance - Selective Side 

Phoenix Cape Canaveral Palo Alto Gaithersburg 

Days Exposure* Days Exposure Days Exposure Days Exposure 

Sample Control 80 160 240 480 80 160 240 480 80 160 240 480 80 160 240 480 

A 0.87 0.87 0.86 0.88 -- 0.88 0.85 0.86 0.86 0.88 0.86 0.87 0.88 0.87 -- 0.87 0.87 

C 0.98 0.97 0.97 0.97 0.97 0.97 0.98 0.98 0.97 0.97 0.97 0.98 0.97 0.97 0.97 0.97 0.97 

D 0.97 0.96 0.95 0.94 0.94 0.95 0.96 0.96 0.94 0.96 0.96 0.96 0.95 0.96 0.96 0.96 0.94 

E 0.95 0.96 0.96 0.96 0.96 0. 96 0. 96 0. 96 . 96 . 96 0. 96 0. 96 0. 96 . 96 0. 96 0. 96 . 96 

F 0.96 0.92 0.92 0.90 0.89 0.93 0.91 0.91 0.89 0.93 0.90 0.90 0.89 0.93 0.92 0.91 0.89 

G 0.93 0.92 0.90 0.92 0.93 0.93 0.93 0.93 -- 0.93 0.93 0.93 -- 0.93 0.93 0.93 -- 

H 0.95 0.96 0.95 0.96 0.94 0.95 0.96 0.96 0.95 0.95 0.96 -- 0.95 0.95 0.96 0.96 0.95 

I 0.88 -- -- 0.87 -- -- -- 0.88 0.87 — -- 0.88 0.86 -- -- 0.87 0.87 

J 0.98 0.97 0.97 0.97 0.96 0.97 0.97 0.98 0.97 0.97 0.97 0.91 0.90 0.97 0.98 0.97 0.97 

L 0.99 0.98 0.97 0.97 0.96 0.98 0.98 0.99 0.97 0.99 0.99 — 0.98 0.99 0.98 -- 0.97 

M 0.94 0.94 0.92 0.91 — 0.96 0.93 0.96 0.95 0.91 0.96 0.92 0.92 0.94 -- 0.93 0.93 

N 0.93 0.76 -- 0.77 -- 0.74 -- 0.74 0.73 0.76 -- 0.78 0.76 0.77 -- 0.77 0.76 

P 0.96 0.95 0.95 0.94 0.94 0.95 0.95 0.95 0.94 0.95 0.94 0.95 0.95 0.95 0.95 0.95 0.94 



* 17000 kJ/sq-m (1500 Btu/sq-ft) Minimum Days 



4b 



Table 3.2.4. Effect of Outdoor Exposure on Absorber Coating Emittance - Selective Side 



Phoen ix 
Days Exposure* 
Sample Control 80 160 240 480 



Cape Canavera I 

Days Exposure 

80 160 240 480 



Pa lo A I to 

Days Exposure 

80 160 240 480 



Ga i thersburg 
Days Exposure 
80 160 240 480 



0.13 0.10 0.11 0.18 0.22 0.10 0.12 0.11 0.17 0.11 0.13 0.14 0.18 0.14 

0.92 0.91 0.92 -- 0.91 0.91 0.91 0.91 0.91 0.91 0.91 0.91 0.91 0.91 

0.07 0.05 0.10 -- 0.07 . 06 . 09 . 06 . 06 0. 05 0. 07 0. 08 0. 07 0.06 

0.87 0.89 0.85 — 0.90 0.89 0.90 0.90 0.90 0.88 0.89 0.89 0.90 0.88 

0.75 0.65 0.61 -- 0.64 0.66 0.66 0.62 0.63 0.67 0.63 0.62 0.62 0.68 

0.86 0.86 0.86 -- 0.85 0.85 0.85 0.85 0.85 0.85 0.85 0.85 0.85 0.85 

0.89 0.87 -- -- 0.86 0.88 0.85 0.86 0.86 0.88 0.89 0.86 0.86 0.88 

0.19 -- -- 0.17 0.16 -- -- 0.18 0.17 -- -- 0.18 0.18 

0.14 0.13 0.15 -- 0.11 0.10 0.11 0.11 0.12 0.12 0.12 0.10 0.09 0.12 

0.29 0.39 -- -- 0.44 0.39 0.42 0.36 0.44 0.27 0.35 0.30 0.31 0.29 

0.10 0.11 0.08 0.08 0.07 0.12 0.10 0.13 0.13 0.08 0.13 0.11 0.09 0.09 

0.51 0.48 -- 0.49 0.50 0.45 — 0.47 0.47 0.47 -- 0.47 0.48 0.46 

0.08 0.07 0.10 -- 0.09 0.07 0.07 0.07 0.07 0.08 0.08 0.09 0.07 0.07 



0.11 0.11 0.11 

0.91 0.91 0.91 

0.06 0.06 0.06 

0.89 0.89 0.90 

0.65 0.63 0.64 

0.85 0.85 0.85 

0.88 0.87 0.86 

- - 0.170.19 

0.11 0.13 0.12 

0.35 -- 0.34 

-- 0.09 0.09 

-- 0.48 0.48 

0.08 0.06 0.07 



* 17000 kJ/sq-m (1500 Btu/sq-ft) Minimum Days 



47 



side of the absorber box described in section 2. Values for specimens exposed on the nonselective 
side are given in tables 3.2.5 and 3.2.6. With the exception of absorber material H, which had a 
slight, but consistent, improvement in emittance from 0.88 to 0.86 after 480 days on the selective 
side of the absorber box, there were no changes in the optical properties of the nonselective 
absorber coupon specimens studied. Material A increased in emittance from about 0.13 to values of 
0.22, 0.17, and 0.18 after 480 days of selective side exposure at the Phoenix, Cape Canaveral, and 
Palo Alto sites, respectively. No significant changes were observed for this material when exposed 
on the selective side at the Gaithersburg site or for the nonselective side at any of the outdoor 
exposure sites. Material D had a slight tendency towards lower absorptance values, decreasing from 
0.97 to 0.94. Material F had decreases in absorptance from 0.96 to 0.89 (selective side) and about 
0.91 (nonselective side) and improvements in emittance from 0.75 to approximately 0.63 (selective 
side) and 0.66 (nonselective side). The major portion of the changes occurred within 80 days of 
exposure. Material J decreased in absorptance from 0.97 to 0.90 and improved in emittance from 0.12 
to 0.09 when exposed on the selective side of the absorber box at Palo Alto for 480 days. Similar 
changes were not observed at the other exposure sites. Material L increased substantially in emit- 
tance from 0.29 to as much as 0.51 with a slight decrease in absorptance. The changes were greatest 
at the Cape Canaveral site and the least at the Palo Alto and Gaithersburg sites. Material N 
decreased in absorptance from 0.93 to as low as 0.73 and improved slightly in emittance from 0.51 to 
as low as 0.45 within 80 days of outdoor exposure and then showed no further changes. Changes in 
the optical properties of materials, I, M, and P were not obvious from examination of the absorptance 
and emittance data. 

3.2.3 Accelerated Outdoor Testing 

Visual Inspection : One of the two samples of absorptive coating material D after exposure equivalent 
to 2 years "real time" had several small spots which, on microscopic evaluation, were found to be pits 
due to corrosion. After the same exposure, one of two samples of material J exhibited several small 
surface spots, one of two samples of material L showed considerable lightening and a light gray 
mottled appearance, and both specimens of material N changed from dark brown to tan. No other changes 
were observed. 

Optical Measurements : Table 3.2.7 presents values for absorptance and emittance for absorber 
materials exposed outdoors in accelerated weathering machines with temperatures simulating stagnation 
conditions. Material C was the only nonselective absorber material exposed since prior "real time" 
outdoor exposure did not cause significant optical property changes in this class of materials. 
Changes similar to those which occurred in the "real time" outdoor exposure occurred for materials 
D, F, L, N, and one of two samples of material A. The optical property changes in material J, which 
were observed for "real time" exposure of coupon specimens only at the Palo Alto site (and not at 
the other three "real time" outdoor exposure sites), were not observed following accelerated outdoor 
exposure. 

3.2.4 Temperature Testing 

Visual Observations : With the exception of material N, which changed from dark brown to light tan 
within 1000 h of exposure at 150°C, none of the other materials showed visual signs of degradation 
after exposure for up to 2000 h at this temperature. Material A darkened considerably after 1000 h 
at 175°C and became a darker brown after 2000 h at this temperature. After 2000 h at 175°C, 
material L had a slight lightening in color and material G took on a slight purplish tinge. 
Material N changed from dark brown to tan within 1000 h of exposure at this temperature. No other 
visual changes were observed. 

Optical Measurements : Table 3.2.8 presents values for absorptance and emittance for absorber 
materials exposed in the laboratory for up to 2000 h at temperatures of 150°C and 175°C. Absorber 
coating materials F, L, and N were the only materials which showed substantial changes within 2000 h 
of exposure at 150°C. These changes were consistent with those reported for the outdoor exposure 
tests and occurred prior to the 1000 h time period at which samples were removed for measurement. 
After 2000 h at 175°C, material A had a substantial increase in emittance from 0.13 to 0.52. 
Materials F, L, and N also showed substantial changes at this temperature. A slight change in 
emittance was observed for material G after 2000 h. 

3.2.5 Temperature and Humidity Testing 

Visual Observations : After 1000 h at 90°C and 95 percent RH, the absorptive coating on material A 
had disintegrated, exposing the galvanized substrate beneath it. There was also some corrosion 
through the galvanized layer; after 2000 h, there was severe corrosion. Material C had no visual 
signs of degradation after 2000 h of exposure. After 1000 h, a considerable amount of the black 

48 



Table 3.2.5. Effect of Outdoor Exposure on Absorber Coating Absorptance - Nonselective Side 

Phoenix Cape Canaveral Palo Alto Gaithersburg 

Days Exposure* Days Exposure Days Exposure Days Exposure 

Sample Control 80 160 240 480 80 160 240 480 80 160 240 480 80 160 240 480 

A 0.87 0.87 0.87 0.87 -- 0.87 0.87 0.88 0.87 0.87 0.86 0.87 0.87 0.88 0.87 0.86 0.86 

C 0.98 0.97 0.97 0.98 -- 0.97 0.97 0.97 -- 0.97 0.97 0.97 -- 0.97 0.97 0.97 -- 

D 0.97 0.96 0.96 0.96 -- 0. 96 0. 95 0. 96 0. 94 0. 96 0. 96 0. 96 0. 95 0. 96 0. 96 0. 94 0. 94 

E 0.95 0.96 0.96 0.96 -- 0.96 0.96 0.96 -- 0.96 0.96 0.96 -- 0.96 0.96 0.96 -- 

F 0.96 0.93 0.93 0.93 -- 0.93 0.92 0.93 0.91 0.93 0.92 0.93 0.92 0.94 0.93 0.93 0.91 

G 0.93 0.92 0.93 0.92 -- 0.93 0.93 0.93 -- 0.93 0.93 0.93 -- 0.93 0.93 0.93 -- 

H 0.95 0.95 0.95 0.96 -- 0.95 0.95 0.96 0.95 0.95 0.95 — 0.95 0.95 0.95 0.95 0.95 
I 0.88 
J 0.98 

L 0.99 0.99 0.99 0.99 -- 0.98 0.97 0.97 0.96 0.99 0.98 -- 0.98 0.98 0.98 0.98 0.97 

M 0.94 0.91 0.95 0.95 -- ' 0.94 0.91 0.92 0.92 0.96 0.94 0.94 0.94 0.95 0.95 0.94 0.93 

N 0.93 -- -- 0.77 -- -- -- 0.75 0.74 -- -- 0.78 0.78 -- -- 0.78 0.77 

P 0.96 0.95 0.95 0.95 -- 0.95 0.95 0.96 0.94 0.96 0.95 0.95 0.95 0.95 0.95 0.95 0.94 

* 17000 kj/sq-m (1500 Btu/sq-ft) Minimum Days 



49 



Table 3.2.6. Effect of Outdoor Exposure on Absorber Coating Emittance - Nonselective Side 



Phoen i x 
Days Exposure* 
Sample Control 80 160 240 480 



Cape Canavera ! 

Days Exposure 

80 160 240 480 



Pa lo Alto 

Days Exposure 

80 160 240 480 



Ga i thersburg 
Days Exposure 
80 160 240 480 



0.14 0.11 0.11 0.12 0.14 0.10 0.13 0.13 0.14 0.14 0.15 0.13 0.11 0.11 0.10 0.10 

0.91 0.91 0.91 0.91 0.91 0.91 0.91 0.91 0.91 0.91 0.90 0.91 0.91 0.91 0.91 0.91 

0.09 0.07 0.06 0.06 0.07 0.05 0.06 0.06 0.07 0.06 0.08 0.06 0.06 0.06 0.06 0.06 

0.88 0.89 0.89 0.90 0.87 0.89 0.89 0.90 0.87 0.89 0.89 0.90 0.87 0.89 0.89 0.89 

0.66 0.66 0.66 0.65 0.66 0.65 0.66 0.66 0.67 0.66 0.67 0.68 0.68 0.67 0.67 0.66 

0.85 0.85 0.85 0.85 0.85 0.85 0.85 0.85 0.85 0.85 0.85 0.85 0.85 0.85 0.85 0.85 

0.88 0.88 0.88 0.87 0.88 0.88 0.88 0.87 0.88 0.88 0.88 0.87 0.88 0.88 0.88 0.87 



0.29 0.32 0.31 0.37 0.44 0.42 0.46 0.51 0.33 0.38 0.34 0.32 0.32 0.30 0.30 0.31 

0.08 0.11 0.10 0.10 0.10 0.08 0.09 0.10 0.13 0.12 0.11 0.10 0.11 0.10 0.09 0.10 

-- 0.49 0.50 -- -- 0.46 0.47 -- -- 0.48 0.49 -- -- 0.47 0.46 

0.07 0.07 0.07 0.07 0.07 0.08 0.07 0.08 0.07 0.07 0.09 0.08 0.07 0.07 0.07 0.07 



A 


0.13 


B 


0.92 


D 


0.07 


E 


0.87 


E 


0.75 


G 


0.86 


H 


0.89 


I 


0. 19 


J 


0. 14 


L 


0.29 


M 


0. 10 


N 


0.51 


P 


0.08 



* 17000 kj/sq-m (1500 Btu/sq-ft) Minimum Days 



50 



Table 3.2.7. Effect of Accelerated Outdoor Exposure on Absorber Coatings 

Absorptance Emittance 

Control Sample 1 Sample 2 Control Sample 1 Sample 2 

Coating 36* 60 120 36 60 120 36 60 120 36 60 120 

A 0.87 0.87 0.87 0.88 0.87 0.87 0.86 0.13 -- -- 0.18 -- -- 0.12 

C 0.97 0.96 0.97 0.97 0.97 0.98 0.97 0.92 -- -- 0.91 -- -- 0.91 

D 0.96 0.96 0.97 0.93 0.95 0.95 0.94 0.07 -- -- 0.07 -- -- 0.07 

F 0.94 0.95 0.92 0.88 0.91 0.91 0.91 0.75 — -- 0.64 -- -- 0.66 

J 0.97 0.97 0.97 0.96 0.97 0.97 0.96 0.14 -- -- 0.14 -- -- 0.12 

L 0.97 0.97 0.97 0.95 0.97 0.97 0.96 0.29 — — 0.47 -- -- 0.36 

M 0.94 0.93 0.94 0.93 0.94 0.94 0.93 0.10 -- -- 0.11 -- -- 0.10 

N 0.91 0.78 0.77 0.73 0.78 0.76 0.74 0.51 -- -- 0.43 -- -- 0.42 

P 0.94 0.95 0.95 0.94 0.94 0.94 0.94 0.08 -- -- 0.08 -- -- 0.07 

* Equivalent Days; 5 Equivalent Days = 1 Month 'Real Time' = 6.625E+5 kJ/sq m 



51 



Table 3.2.8. Effect of Temperature on Absorber Coatings 



Temperature : 






150 C 








175 


C 






Ab 


sorptance 


Em i ttance 




Absorptance 


Emi ttance 


Sample Control 


1000* 


2000 


Contro I 


1000 


2000 


1000 


2000 


1000 


2000 


A 


0.87 


0.86 


0.86 


0. 13 


0.09 


0. 13 


0.87 


0.91 


0. 13 


0.52 


C 


0.96 


0.97 


0.97 


0.92 


0.91 


0.92 


0.97 


0.97 


0.91 


0.91 


D 


0.97 


0.96 


0.96 


0.07 


0.06 


0.06 


0.95 


0.96 


0.07 


0.07 


E 


0.95 


0.96 


0.96 


0.87 


0.86 


0.87 


0.96 


0.96 


0.89 


0.89 


F 


0.96 


0.91 


0.90 


0.75 


0.66 


0.65 


0.89 


0.89 


0.67 


0.70 


G 


0.93 


0.93 


0.93 


0.86 


0.85 


0.85 


0.94 


0.95 


0.86 


0.82 


H 


0.95 


0.96 


0.96 


0.89 


0.88 


0.88 


0.95 


0.95 


0.88 


0.87 


1 


0.88 


-- 


-- 


0. 19 


0.19 


-- 


0.87 


-- 


0. 18 


-- 


J 


0.98 


0.98 


0.98 


0. 14 


0. 12 


0. 12 


0.97 


0.97 


0. 13 


0. 13 


L 


0.99 


0.99 


0.99 


0.29 


0.37 


0.35 


0.98 


0.99 


0.41 


0.38 


M 


0.94 


0.95 


0.91 


0.10 


0. 12 


0.08 


0.93 


0.92 


0. 12 


0. 12 


N 


0.93 


0.85 


-- 


0.51 


0.49 


-- 


0.80 


-- 


0.50 


-- 


P 


0.96 


0.95 


0.95 


0.08 


0.07 


0.06 


0.95 


0.95 


0.08 


0.10 



* Hours Exposure 



52 



chrome absorptive coating had flaked off absorber material D, exposing the nickel layer beneath it, 
and there was some pitting and corrosion of the steel substrate through the nickel layer in the 
areas where the flaking occurred. This pitting and corrosion became more severe after 2000 h; however, 
it did not have the same appearance as that which occurred on the absorbers of full-size collectors. 
Coating E lightened to mid-gray after 1000 h at 90°C and 95 percent RH and remained the same color 
after 2000 h. Several small mid-gray specks appeared on the coating of material F after 1000 h of 
exposure which became somewhat larger after 2000 h. There was severe surface etching of the porce- 
lain enamel coating of material G after 1000 h and some rust spots; this remained about the same 
after 2000 h of exposure. Extensive broken paint blisters were observed in coating H after 1000 h 
and 2000 h of exposure. Coating I had no visual signs of degradation after 1000 h of exposure. A 
considerable amount of light gray specks and streaking was observed in coating J after 1000 h and 
2000 h of exposure. Material L was severely corroded and took on a white, mottled gray appearance 
after 1000 h and 2000 h at 90°C and 95 percent RH. The absorptive coatings of materials M and N were 
removed, exposing the bare metal substrate after 1000 h of temperature and moisture exposure. There 
were large gray mottled areas on the coating of material P after 1000 h of exposure and the coating 
appeared to be washed away. 

Optical Measurements : Table 3.2.9 presents values for absorptance and emittance for absorber 
materials exposed in the laboratory for up to 2000 h at 90°C and 95 percent RH. The nonselective 
absorber materials (C, E, G, and H) were not significantly affected by this exposure. With the 
exception of material I, which was only exposed to these conditions for 1000 h and material F, all 
of the selective absorber materials (A, D, J, L, M, N and P) had substantial (>0.03) changes in 
their optical properties. 

3.2.6 Temperature and Xenon Arc Radiation Testing 

Visual Observations : Absorber coating materials A, C, D, E, H, I, J, M, and P had no visual signs 
of change after exposure to xenon arc radiation at 90°C for up to 2000 h. Material F changed from 
black to dark gray after 2000 h of exposure. There was a dull gray surface film on material G, 
after 1000 h and 2000 h of exposure, which could be wiped off. Material L lightened to a medium 
gray from a dark gray after 1000 h and 2000 h of exposure and material N changed from dark brown to 
tan. 

Optical Measurements : Table 3.2.9 presents values for absorptance and emittance for absorber 
materials exposed in the laboratory for up to 2000 h in conjunction with xenon arc radiation simu- 
lating the solar spectrum. Materials F, I, L, N, and P showed substantial changes in their optical 
properties as a result of this exposure. 

3.2.7 Thermal Cycling 

Visual Observations : White spots, similar in appearance to those observed on full-size collectors, 
were observed on absorber coating material A after 15 cycles from -10°C to 175°C. This whitening 
became progressively more severe after 30 cycles. After thermal cycling, material D had small spots 
which looked like the early stages of corrosion observed on full-size collectors. Light specks 
appeared on the surface of the material J after 15 and 30 cycles. Material L had a mottled light to 
medium gray appearance after 15 cycles and the surface of material N changed from dark brown to tan 
after 15 cycles. Visual changes were not observed for materials C, E, F, G, H, I, M, and P. 

Optical Measurements : Table 3.2.10 presents values for absorptance and emittance for absorber 
materials exposed in the laboratory for up to 30 cycles from -10°C to 175°C. Materials A, F, L, M. 
and N had significant changes in their optical properties. 

3.2.8 Comparison and Assessment of Test Procedures 

The test boxes used for the outdoor "real time" exposure of absorber coupon specimens appear to be 
a good way of exposing a large number of samples to determine their thermal stability under stagna- 
tion conditions. However, the boxes, which were carefully constructed and designed to be watertight, 
did not have the moisture and condensation problems that were observed for virtually all of the 
full-size collectors. The problem of how to determine the proper test conditions for assessing the 
moisture stability of absorber materials is very complex, i.e., the presence of moisture on the 
inner surface of the glazing of a collector that is stagnating on a clear day does not mean that the 
relative humidity in the vicinity of the absorber is anywhere near as high as that in the vicinity 
of the glazing since absorber temperatures are much higher than cover temperatures. In addition, the 
presence of porosity in many absorptive coatings means that moisture can condense in these pores at 
humidities lower than 100 percent RH. It is more likely that moisture would condense out on the 
absorber at night, when it is cool, rather than in the daytime. The thermal cycling test was the test 

53 



Table 3.2.9. Effect of Temperature & Humidity and Temperature & Xenon Arc Radiation on 
Absorber Coatings 

Temperature & Humidity, 90 C & 95% RH Temperature & Radiation, 90 C & Xenon Lamp 
Absorptance Emittance Absorptance Emittance 

Sample Control 1000* 2000 Control 1000 2000 1000 2000 1000 2000 



A 


0.87 


0.73 


0.84 


0. 13 


0.70 


0.53 


C 


0.98 


0.97 


0.97 


0.92 


0.91 


0.91 


D 


0.97 


0.95 


0.95 


0.07 


0. 13 


0.19 


E 


0.95 


0.92 


0.92 


0.87 


0.88 


0.89 


F 


0.96 


0.93 


0.93 


0.75 


0.76 


0.77 


G 


0.93 


0.93 


0.94 


0.86 


0.89 


0.87 


H 


0.95 


0.94 


0.94 


0.89 


0.90 


0.90 


1 


0.88 


0.88 


-- 


0. 19 


0. 17 


-- 


J 


0.98 


0.96 


0.96 


0. 14 


0.22 


0.26 


L 


0.99 


0.85 


0.81 


0.29 


0.81 


0.85 


M 


0.94 


0.59 


0.54 


0. 10 


0.62 


0.65 


N 


0.93 


0.50 


-- 


0.51 


0.81 


-- 


P 


0.96 


0.95 


0.95 


0.08 


0. 12 


0. 14 



0.86 


0.87 


0. 10 


0. 11 


0.97 


0.98 


0.91 


0.91 


0.96 


0.96 


0.06 


0.06 


0.96 


0.97 


0.89 


0.89 


0.90 


0.89 


0.73 


0.71 


0.94 


0.94 


0.86 


0.86 


0.95 


0.96 


0.87 


0.87 


0.85 


-- 


0.15 


-- 


0.98 


0.98 


0. 12 


0. 11 


0.98 


0.90 


0.37 


0.39 


0.94 


0.95 


0. 10 


0. 11 


0.79 


-- 


0.45 


-- 


0.95 


0.96 


0. 11 


0.32 



* Hours Exposure 



54 



Table 3.2.10. Effect of Thermal Cycling on Absorber Coatings 



Absorptance Emittance 

Thermal Cycling (Simulated Daily Cycles)* 
Sample Control 5 15 30 Control 5 15 30 



A 


0.87 


-- 


0.87 


0.87 


0. 13 


0.16 


0.25 


C 


0.98 


-- 


0.97 


0.97 


0.92 


0.91 


0.91 


D 


0.97 


-- 


0.95 


0.96 


0.07 


0.06 


0.06 


E 


0.95 


-- 


0.96 


0.96 


0.87 


0.86 


0.85 


F 


0.96 


-- 


0.91 


0.91 


0.75 


0.64 


0.65 


G 


0.93 


-- 


0.92 


0.93 


0.86 


0.85 


0.85 


H 


0.95 


-- 


0.95 


0.95 


0.89 


0.88 


0.88 


I 


. 88 


-- 


0.88 


-- 


0. 19 


0.17 


-- 


J 


0.98 


-- 


0.97 


0.97 


0. 14 


0.16 


0. 12 


L 


0.99 


-- 


0.95 


0.94 


0.29 


0.45 


0.43 


M 


0.94 


-- 


-- 


0.94 


0.10 


-- 


0.14 


N 


0.93 


-- 


0.85 


-- 


0.51 


0.53 


-- 


P 


0.96 


-- 


0.95 


0.95 


0.08 


0.09 


0.09 



10 C to 175 C 



55 



which most closely simulated the types of corrosion and other changes that were observed in full-size 
collectors. In this test, coupon specimens were removed from a chamber at -10°C and allowed to 
equilibrate at room temperature prior to being placed in an oven at 177 °C. During this equilibration 
process, moisture condensed on the test specimen surfaces; this most likely led to the corrosion 
observed. The accelerated outdoor test, in which test specimens were exposed to concentrated solar 
radiation and stagnation temperatures, appeared to provide results compariable to the "real time" 
coupon specimen exposure; but it was not obvious that the concentrated solar radiation accelerated 
the photolytic degradation of absorber materials. Laboratory testing at stagnation temperatures 
brought out a number of thermal stability problems; however, these problems and a number of additional 
ones were also brought out by the thermal cycling test. The temperature and humidity testing caused 
corrosion problems different from those observed in "real time" exposure and is believed to be unduly 
severe. Exposure to temperature and xenon arc radiation made evident some appearance changes not 
observed with other tests. The importance of this test will be determined to a large extent by the 
spectral transmittance of the collector glazing used in conjunction with the absorber. 

3.3 ADDITIONAL COLLECTOR-LEVEL OBSERVATIONS 

During the outdoor exposure of the full-size collectors, visual observations were made to identify 
potential problems. Condensed moisture on the Inner surfaces of glazing materials was a common 
occurrence, especially after periods of rainfall. In several cases, there were obvious openings in 
the collector cases. The corners of the cases of several of the collectors used in the test program 
were found to be unsealed on disassembly following outdoor exposure and testing. In addition, 
several of the glazing gasket designs used would allow ponding of water between the glazing and the 
gasket on the lower part of the collector during a rainstorm. It is unlikely that the water conden- 
sation observed is due only to normal collector breathing and cooling down at night in view of dif- 
ferences observed in the corrosion of duplicate collectors exposed outdoors at the same exposure 
sites. All of the exposed collectors had visible evidence of the buildup of outgassing products on 
their glazings; however, these products did not appear to substantially affect the collector thermal 
performance as will be discussed in section 4. As discussed in section 3.2.2, all of the paints 
used as absorptive coatings gave off condensible outgassing products when exposed in the absorber 
test box as coupon specimens. Another probable source of outgassing is the insulation used in 
several of the collectors as will be discussed later in this section. Outgassing was especially 
obvious in collector H where the buildup of condensible material on the inner glazing resulted in a 
change in transmittance from an original value of 0.96 to as low as 0.79. The amount of change mea- 
sured varied from collector to collector and depended on the location within the collector from 
which the sample was taken. The fluorinated (ethylene propylene) copolymer film used as an inner 
glazing in collector H and as a heat trap in collector C tended to sag and touch the absorber plates 
when the collectors were heated by the sun. Support wires were used in collector G to prevent 
this from happening. The film did not appear to be affected by a few years of contact with the 
absorbers of the Type C collectors, which reached temperatures as high as 160°C under stagnation 
conditions. Absorber plate buckling was also observed in several collector types; however, it did 
not appear to be serious enough to cause failure. There was also evidence of cracking in the rubber 
grommets used to seal the absorber plate inlet and outlet tubes. 

Disassembly of the solar collectors following the completion of outdoor stagnation exposure for 480 
days revealed a number of problems. The paper facing material used on mineral wool insulation in 
collector A charred badly on the side in contact with the absorber plate. There was a considerable 
amount of browning and shrinkage of the foam insulation used in collector C, despite the fact that 
there was a 2.5 cm layer of glass fiber insulation between the absorber plate and the foam material. 
The binder in the top layer of glass fiber insulation turned from yellow to tan. The binder in the 
glass fiber insulation used in collector B turned from yellow to brown in the layer closest to the 
absorber plate. The binder was completely burned out of the glass fiber insulation used in collec- 
tor D to a depth of about 2.5 cm on the side closest to the absorber plate. Absorber plate stagna- 
tion temperatures of 220°C were measured for this collector on clear days. The foil-faced foam 
insulation board used in collectors E, G, and H, and in collector F with a top layer of glass 
fiber insulation 2.5 cm thick showed little evidence of change. 



56 



4. RESULTS AND DISCUSSION: COLLECTOR THERMAL PERFORMANCE AND EXPOSURE STUDIES 

The sections which follow: 1) summarize previously published research findings resulting from this 
test program, 2) present the collector thermal efficiency test results obtained during the test 
program and analyze possible sources of the scatter observed in these measurements, and 3) analyze 
the sensitivity of collector thermal efficiency and absorber plate stagnation temperature measure- 
ments to changes in collector materials performance. 

4.1 SUMMARY OF PREVIOUSLY REPORTED RESEARCH FINDINGS OF THIS PROGRAM 

The research findings summarized in the following sections are discussed in detail in the 
publications referenced. 

4.1.1 Collector Thermal Performance Test Data Uncertainty 

Statistical analyses of the thermal performance data sets obtained on eight liquid-heating, 
flat-plate solar collectors at four test sites broadly dispersed in the U.S. have confirmed that the 
total experimental uncertainty is about the same as the probable estimate of random error predicted 
for the use of ASHRAE Standard 93-77 [24, 25]. Measurement error is believed to be the major con- 
tributor to the "within" test site variability (repeatibility) , and environmental effects a signifi- 
cant additional factor influencing the slope (loss coefficient) data obtained "between" test sites 
(reproducibility). No evidence of product variability was discerned for the various collector types. 

On the basis of first-order curve fits of the data points, an average "within" test site coefficient 
of variation, percent cv r , of 2.1 percent and "between" test site coefficient of variation, percent 
cvr, of 2.4 percent was calculated for the intercept [F^(Ta) e ]. 

Similar data for the overall heat loss coefficient, FrUl, indicated a "within" (percent cv r ) 
coefficient of variation value of 5.93 percent and a "between" (percent cvr) of 8.37 percent. A 
coefficient of variation (percent cvr) for incident angle modifier (IAM) data of 34 percent was 
found based upon measurements at three test sites. 

The data uncertainties, when used in appropriate analytical models, resulted in the following 
application uncertainties: 

• Material degradation equivalent to about a 0.10 change in cover transmittance or absorber 
solar absorptance and emittance 

• Variations in collector all-day thermal output of from i 17 to ± 68 percent for specific 
winter operating conditions resulting in significant ranking changes 

• Variations in residential annual heating and domestic hot water solar fraction of i 6 to 7 
percent. 

4.1.2 Analysis of Measurement and Calculation Procedures for Incident Angle Modifiers for Flat-Plate 
Solar Collectors 

The test procedure used to determine incident angle modifiers (IAM) for flat-plate collectors was 
shown to give an IAM curve with a large uncertainty [26, 27]. The problem results from measuring 
collector efficiency at non-normal incident angles where the measurement uncertainty is of the same 
order as the efficiency reduction attributable to these off-normal angles. The effects of side and 
end shading of the absorber are of the same order of importance as the effects of non-normal angles 
on the optical properties of the cover assembly. Consequently, even flat-plate collectors can have a 
significant bidirectional angular response. 

Comparing the advantages and limitations of analytical versus experimental procedures for determining 
incident angle modifiers for flat plate collectors of simple geometry, analytical methods are con- 
sidered to be as adequate as current experimental procedures and considerably less expensive. A 
simplified analytical procedure for calculating the IAM for these type collectors is described. 
Further consideration is needed, however, for collectors with additional directional characteristics, 
such as convex covers, honeycomb grid convection suppressors, and nontracking radiation 
concentration features. 



57 



4.1.3 Evaluation of Absorber Stagnation Temperature as an Indicator of Changes in Solar Collector 
Materials Performance 

Results obtained from nearly steady-state measurements (actually, time-average results for a 
relatively short duration) showed that the technique based on measuring the absorber stagnation 
temperature rise above ambient resulting from a given solar irradiance is at least as, and possibly 
more, sensitive than thermal efficiency measurements for detecting small changes in material proper- 
ties with a much simpler test apparatus and experimental procedure [30, 31]. While the measured 
results using the new method appear reliable and less expensive to obtain, problems were identified 
which can be attributed to the strong effects on measured absorber temperature of transients result- 
ing from changes in solar irradiance and environmental conditions. Short term transients in the 
solar irradiance profile, such as would be caused by intermittent cloud cover, and in other environ- 
mental parameters can reduce the reproducibility of the data significantly. A solar irradiance 
simulator could solve the problems arising from such transients and nonrepeating environmental con- 
ditions. A limitation associated with simulators, however, is the possible sensitivity of material 
properties to the spectral distribution of the irradiance simulator. 

A subsequent investigation [32] showed that an all-day integration method is a viable approach for 
outdoor experimental determination of degradation in the thermal performance of solar collector 
materials. This method is at least as accurate as the energy output method, provided that appropriate 
limits are placed on variations in environmental conditions (primarily wind), and much simpler to 
perform experimentally. The all-day integration technique resolves problems associated with the 
method based on steady-state temperatures such as short-term transients in solar irradiance and 
wind speed, which are serious limitations. The graphical presentation of data resolves the 
difficulties of comparing results obtained on days with different levels in solar irradiation. 

4.1.4 Comparison of Solar Simulator and Outdoor ASHRAE Standard 93 Thermal Performance Tests 

Standard test methods for determination of solar collector thermal performance permit the use of 
solar simulators. An evaluation of available outdoor and solar simulator data [28] showed that the 
thermal efficiency of flat-plate solar collectors can be substantially higher when measured in a 
solar simulator. The data and analytical studies indicated that heated optics and a relatively large 
view factor can cause excessive infrared radiation exchange with the collector cover and result in 
higher efficiencies for some collectors. Modifications to the simulator to change the spec- 
tral distribution, incident angle and direct/diffuse ratio are possible but careful monitoring of 
the uniformity, spectral distributions, and effective environmental radiance temperature is necessary 
to maintain consistent test conditions. 

A more extensive experimental and analytical evaluation of solar simulator characteristics was 
recommended to determine the minimum requirements of adequate simulation and to provide a better 
understanding of the effect of individual environmental parameters experienced in outdoor testing. 



58 



4.2 OUTDOOR SLOPE AND INTERCEPT DATA 

In this section, collector thermal performance results are presented and compared. The bases of 
comparison are the intercept and (negative) slope of the linearized curves which correlate measured 
efficiency values. 

The most commonly used method of characterizing collector performance is in terms of the thermal 

efficiency curve. The useful energy gain may be determined from a Hottel-Whillier-Bliss analysis 

for collectors of the type used in the present study. Based on this theory, the thermal efficiency 
may be expressed as 

n = F R (xa) - F R U L [t.-t a ]/G (4.2.1) 

If the parameters F (xa) and F U are assumed constant, the efficiency is a linear function of the 

parameter [t.-t ] /G where F (xa) is the intercept and -F U is the slope of the efficiency curve. 
i a R R L 

The assumptions of constant F (xa) and F U are reasonable for correlating efficiency test results. 
The transmittance-absorptance product, (xa) , is a measure of collector optical characteristics and 
is relatively insensitive to the test environment. The overall loss coefficient U depends more 
strongly on environmental factors such as wind speed, ambient and sky temperature, and mean absorber 
plate temperature. The heat removal factor F is a weak function of U and, consequently, depends 
somewhat on collector temperature. Previous investigations [26, 39, 40] have shown, however, that 
any errors inherent in assuming constant F (xa) and constant F U are generally overshadowed by 
experimental error and variations caused by different test environments. In a later section the 
suitability of linearized efficiency curves for comparing results is further validated. 

The values reported for the intercept and slope of the efficiency curves for all tests conducted in 
the program are presented in table 4.2.1. The collector type, test site, and test series, respec- 
tively, are designated by the codes in the first column. The other eight column pairs from left to 
right show the performance parameters for the initial (0- or 3-day) through final tests. This 
tabulation of measured results is the data base used for all investigations of possible thermal degra- 
dation in the collectors. In order to investigate possible trends more clearly, the values of the 
parameters F (xa) and F U are presented in bar graph type plots for each retest . These results are 
shown in figures 4.2.1 through 4.2.24. The parameters are normalized with initial (0-day) test 
values for test series 1, 3, and 4. For the series 2 tests, the results are normalized with initial 
values which were obtained after 3 days of exposure. 

Several observations follow from an examination of the slope and intercept data. Few, if any, 
general conclusive statements can be made with regard to long-term degradation trends. The figures 
show an apparent random scatter in results of about the same order of magnitude as any general 
change in performance. While the plots of F U vs exposure exhibit far more scatter than the 
corresponding plots for F (xa) , even the latter show significant and inconsistent variations within 
and between test sites, collectors, and retests. 

A greater scatter in the F U plots is expected because this parameter is affected more strongly by 
environmental and test conaitions than is F (xa) . Additionally, visual inspection reports from the 
participating laboratories suggest that the insulation in some collectors might experience a curing 
process which would affect F U . On the other hand, visual inspections suggested some insulations 
were degraded by the presence of moisture within the collector. The effect of wind speed on the 
loss coefficient is strong. Small differences between laboratories in measurement location and wind 
conditions preceding the actual test can affect measured efficiency results. 

The parameter F (xa) does not exhibit as strong an environmental dependence as FrU*l and should be 
a more repeatable measure of collector thermal performance. As a result, more emphasis is placed on 
investigating the changes in F (xa) with exposure as a characteristic index of performance. In a 
later section, the effect of variations in environmental conditions on both slope and intercept are 
investigated. 

Considerine series 1 and 2 results, only collectors F, G, and H show a distinct systematic decrease 
in the intercept parameter. These collectors all have plastic or glass fiber reinforced plastic (FRP) 
covers. Collector B, in fact, shows a general improvement in the intercept parameter with exposure. 
This collector with glass covers and a flat-black absorber is evidently quite stable. As expected, 
there are no general trends in the slope parameter F U which is more sensitive to changes in 
environmental conditions. 

An examination of figures 4.2.1 through 4.2.24 shows one other apparent trend in the intercept 
parameters. This parameter tends to peak between 30 and 60 exposure days and thereafter decrease 

59 



Table 4.2.1 Summary of all Reported F (ta) and F U T Values 

K R L 



COLLECTOR 














RETEST 


DAYS 


















) 




3 




15 




30 




50 


120 


240 


480 




FRTA 


FRUL 


FRTA 


FRUL 


FRTA 


FRUL 


FRTA 


FRUL 


FRTA 


FRUL 


FRTA 


FRUL 


FRTA FRUL 


FRTA 


FRUL 


A-1-1 


0.602 


4.562 


0.593 


4.308 


N/A 


N/A 


0.599 


4.023 


0.606 


3.978 


0.590 


4.060 


0.591 4.381 


0.583 


4.366 


A-1-2 


N/A 


N/A 


0.596 


4.440 


N/A 


N/A 


0.605 


4.092 


0.628 


4.318 


0.613 


3.960 


0.596 4.505 


0.600 


4.440 


A- 1-3 


0.622 


4.549 


N/A 


N/A 


0.623 


3.910 


0.636 


4.244 


0.623 


4.987 


0.598 


3.766 


0.596 4.585 


N/A 


N/A 


A-1-4 


0.601 


4.429 


N/A 


N/A 


0.610 


4.148 


0.603 


4.307 


0.591 


5.107 


N/A 


N/A 


N/A N/A 


N/A 


N/A 


A-2-1 


0.609 


5.250 


0.609 


4.850 


0.610 


4.610 


0.623 


4.280 


0.602 


4.550 


0.607 


4.870 


0.617 4.392 


0.623 


»4.679 


A-2-2 


N/A 


N/A 


0.605 


4.980 


0.610 


5.100 


0.628 


4.11-0 


0.626 


5.630 


0.607 


5. 190 


0.588 4. 184 


0.600 


•4.394 


A-2-3 


0.612 


4.490 


N/A 


N/A 


0.637 


5.120 


0.622 


4.560 


0.610 


5.150 


0.610 


5.020 


0.604 4.305 


N/A 


N/A 


A-2-4 


0.581 


4.427 


N/A 


N/A 


0.629 


4.967 


0.617 


4.893 


0.608 


5.113 


N/A 


N/A 


.N/A N/A 


N/A 


N/A 


A-3-1 


0.627 


3.910 


0.633 


3.810 


0.605 


3.600 


0.628 


4.310 


0.629 


4.120 


0.641 


3.980 


0.641 4.360 


0.646 


3.901 


A-3-2 


N/A 


N/A 


0.612 


3.660 


0.636 


4.450 


0.612 


4.390 


0.614 


4.250 


0.629 


4.030 


0.626 4.420 


0.607 


3.544 


A-4-1 


0.611 


4.349 


0.660 


5.167 


0.614 


4.389 


0.601 


4.583 


0.598 


3.805 


N/A 


N/A 


0.617 4.024 


0.643 


3.974 


A-4-2 


N/A 


N/A 


0.655 


5.445 


0.595 


4.571 


0.604 


4.702 


0.598 


4.350 


N/A 


N/A 


0.607 4.430 


0.654 


4.505 


8-1-1 


0.654 


5.460 


0.654 


5.437 


N/A 


N/A 


0.649 


5.001 


0.670 


5.078 


0.670 


5.490 


0.630 5.022 


0.659 


5.479 


B-l-2 


N/A 


N/A 


0.640 


5.428 


N/A 


N/A 


0.661 


5.306 


0.668 


5.074 


0.669 


5. 170 


0.656 5.594 


0.651 


5.440 


8-1-3 


0.652 


5.476 


N/A 


N/A 


0.652 


5.350 


0.688 


5.420 


0.656 


5.624 


0.647 


5.173 


0.648 5.610 


N/A 


N/A 


B-1-4 


0.666 


5.459 


N/A 


N/A 


0.669 


5.536 


0.681 


5.539 


0.636 


5.038 


N/A 


N/A 


N/A N/A 


N/A 


N/A 


B-2-1 


0.654 


5.380 


0.640 


5.384 


0.665 


6.105 


0.648 


5.648 


0.673 


5.705 


0.666 


5.780 


0.668 5.254 


0.683 


»5.472 


B-2-2 


N/A 


N/A 


0.646 


5.696 


0.659 


6.209 


0.651 


5.693 


0.669 


5.636 


0.669 


5.770 


0.688 5.891 


0.685 


»5.349 


B-2-3 


0.648 


5.588 


N/A 


N/A 


0.666 


5.852 


0.668 


6.011 


0.662 


5.364 


0.668 


5.320 


0.671 5.608 


N/A 


N/A 


B-2-4 


0.648 


5.730 


N/A 


N/A 


0.651 


5.737 


0.655 


5.869 


0.653 


5.824 


N/A 


N/A 


N/A N/A 


N/A 


N/A 


B-3-1 


0.652 


5.433 


0.653 


5.390 


0.683 


5.460 


0.656 


5.174 


0.663 


5.417 


0.674 


4.990 


0.686 5.540 


0.689 


5.200 


B-3-2 


N/A 


N/A 


0.636 


5.180 


0.654 


5.150 


0.653 


5.159 


0.668 


5.520 


0.675 


5.060 


0.694 5.750 


0.690 


5.349 


8-U-l 


0.672 


5.212 


0.696 


5.865 


0.709 


5.746 


0.719 


5.723 


0.662 


4.616 


0.651 


4.719 


0.688*5.440 


0.673 


5.007 


B-4-2 


N/A 


N/A 


0.680 


6.200 


N/A 


N/A 


0.687 


5.383 


0.654 


4.452 


0.647 


4.753 


0.705*5.881 


0.665 


4.984 


C-1-1 


0.545 


4.377 


0.528 


4.076 


N/A 


N/A 


0.528 


3.889 


0.534 


3.627 


0.531 


4.059 


0.511 3.834 


0.492 


3.838 


C-1-2 


N/A 


N/A 


0.520 


3.969 


N/A 


N/A 


0.538 


3.915 


0.529 


3.717 


0.527 


4.172 


0.512 3.591 


0.493 


3.929 


C-1-3 


0.538 


3.908 


N/A 


N/A 


0.524 


4. 105 


0.522 


3.566 


0.530 


4.283 


0.524 


3.714 


0.510 4.457 


N/A 


N/A 


C-1-U 


0.540 


4.465 


N/A 


N/A 


0.496 


3.765 


0.485 


3.345 


0.465 


3.828 


N/A 


N/A 


N/A N/A 


N/A 


N/A 


C-2-1 


0.524 


4.531 


0.522 


4.324 


0.541 


4.578 


0.542 


4.488 


0.539 


4.098 


0.541 


4.294 


0.537 3.942 


0.529 


♦3.813 


C-2-2 


N/A 


N/A 


0.511 


4.268 


0.524 


4.513 


0.525 


3.929 


0.517 


3.968 


0.512 


4.173 


0.498 3.840 


0.501*3.983 


C-2-3 


0.557 


4.739 


N/A 


N/A 


0.552 


4.105 


0.549 


4.586 


0.546 


4.456 


0.552 


4. 131 


0.557 4.420 


N/A 


N/A 


C-2-4 


0.541 


4.802 


N/A 


N/A 


0.540 


4.604 


0.540 


4.549 


0.521 


4.022 


N/A 


N/A 


N/A N/A 


N/A 


N/A 


C-3-1 


0.549 


4.228 


0.530 


3.919 


0.540 


3.889 


0.531 


3.867 


0.528 


3.799 


0.541 


3.747 


0.536 3.776 


0.556 


3.907 


C-3-2 


N/A 


N/A 


0.517 


4.044 


0.531 


3.782 


0.530 


4.026 


0.523 


3.725 


0.543 


3.600 


0.538 3.839 


N/A 


N/A 


C-4-1 


0.552 


4. 151 


0.573 


4.849 


0.540 


3.975 


0.546 


4.713 


0.532 


3.833 


0.524 


3.311 


0.507*3.832 


0.483 


2.880 


C-4-2 


N/A 


N/A 


0.549 


4.060 


0.552 


3.992 


0.532 


4.537 


0.525 


4.491 


0.548 


3.958 


0.518*3.311 


0.540 


3.389 


D-l-1 


0.648 


3.468 


0.629 


3.152 


N/A 


N/A 


0.632 


2.654 


0.642 


2.648 


0.644 


3.180 


0.620 2.969 


0.617 


3.060 


0-1-2 


N/A 


N/A 


0.633 


3.179 


N/A 


N/A 


0.639 


3.059 


0.649 


2.790 


0.666 


3.220 


0.635 2.781 


0.622 


3.015 


0-1-3 


0.626 


2.833 


N/A 


N/A 


0.613 


2.982 


0.637 


3.174 


0.618 


3.090 


0.634 


2.902 


0.619 3.229 


N/A 


N/A 


D-1-4 


0.640 


3.200 


N/A 


N/A 


0.623 


2.968 


0.632 


2.790 


0.605 


3.013 


N/A 


N/A 


N/A N/A 


N/A 


N/A 


0-2-1 


0.640 


3.410 


0.650 


3.200 


0.647 


3.490 


0.634 


3.250 


0.637 


3.300 


0.651 


3.820 


0.610 3.789 


0.657*3.498 


0-2-2 


N/A 


N/A 


0.659 


3.010 


0.656 


3.290 


0.646 


3.470 


0.641 


3.400 


0.647 


3.720 


0.625 3.164 


0.635*3.578 


0-2-3 


0.655 


3.450 


N/A 


N/A 


0.660 


3.400 


0.663 


3.280 


0.640 


2.950 


0.662 


3.280 


0.636 3.564 


N/A 


N/A 


0-2-4 


0.636 


3.385 


N/A 


N/A 


0.658 


3.434 


0.645 


3.364 


0.643 


3.485 


N/A 


N/A 


N/A N/A 


N/A 


N/A 


0-3-1 


0.647 


2.555 


0.641 


2.410 


0.619 


2.610 


0.639 


2.940 


0.648 


3.040 


0.632 


2.425 


0.634 2.740 


0.650 


3. 140 


0-3-2 


N/A 


N/A 


0.635 


3.640 


0.629 


2.620 


0.652 


3.100 


0.657 


3.110 


0.646 


2.487 


0.645 2.870 


0.661 


3. 134 


0-4-1 


0.636 


3.055 


0.657 


3.276 


0.688 


3.946 


0.691 


3.583 


0.642 


2.953 


0.616 


3.021 


0.656*3.353 


0.633 


2.950 


D-4-2 


N/A 


N/A 


0.670 


3.384 


0.678 


3.390 


0.664 


3. 100 


0.645 


2.646 


0.625 


2.396 


0.647*3.281 


0.634 


2.822 


£-1-1 


0.613 


7.465 


0.605 


5.997 


N/A 


N/A 


0.624 


5.866 


0.625 


6.260 


0.600 


5.770 


0.585 6. 158 


0.571 


6.479 


£-1-2 


N/A 


N/A 


0.598 


6.443 


N/A 


N/A 


0.624 


6.269 


0.619 


6.129 


0.605 


5.480 


0.596 6.645 


0.569 


6.359 


E-1-3 


0.599 


6.801 


N/A 


N/A 


0.595 


6.063 


0.609 


6.222 


0.616 


6.470 


0.604 


6.680 


0.578 6.032 


N/A 


N/A 


E-1-H 


0.590 


6. 149 


N/A 


N/A 


0.619 


5.805 


0.618 


5.574 


0.595 


7.101 


N/A 


N/A 


N/A N/A 


N/A 


N/A 


E-2-1 


0.588 


6.488 


0.576 


5.816 


0.605 


6.700 


0.602 


6.233 


0.595 


6.679 


0.590 


6.750 


0.612 6.185 


0.600*6.546 


E-2-2 


N/A 


N/A 


0.619 


6.580 


0.630 


7.639 


0.616 


6.515 


0.613 


6.769 


0.612 


6.450 


0.611 6.737 


0.601*6.633 


E-2-3 


0.602 


7.027 


N/A 


N/A 


0.614 


6.963 


0.610 


5.970 


0.609 


6.780 


0.605 


6.810 


0.620 6.330 


N/A 


N/A 


E-2-4 


0.603 


6.045 


N/A 


N/A 


0.595 


6.448 


0.588 


6.234 


0.569 


6.653 


N/A 


N/A- 


N/A N/A 


N/A 


N/A 


E-3-1 


0.621 


6.306 


0.596 


5.630 


0.602 


6.600 


0.616 


6.547 


0.621 


5.820 


0.625 


6.820 


0.642 5.945 


0.657*6. 179 


E-3-2 


N/A 


N/A 


0.595 


5.584 


0.598 


6.560 


0.626 


6.696 


0.617 


5.830 


0.633 


6.880 


0.637 5.871 


0.65.1*6.332 


E-4-1 


0.619 


6.257 


0.598 


5.990 


0.597 


6.331 


0.594 


6.172 


0.618 


6.451 


N/A 


N/A 


0.611 5.837 


0.619 


5.050 


E-4-2 


N/A 


N/A 


0.602 


6.217 


0.583 


5.996 


0.585 


5.792 


0.614 


6.451 


N/A 


N/A 


0.610 6.016 


0.618 


4.937 


F-1-1 


0.665 


6.504 


0.648 


5.152 


N/A 


N/A 


0.668 


5.152 


0.664 


5.859 


0.633 


5.590 


0.616 5.655 


0.631 


6.433 


F-1-2 


N/A 


N/A 


0.638 


6.728 


N/A 


N/A 


0.659 


5.798 


0.644 


5.528 


0.642 


5.627 


0.615 5.291 


0.621 


6.200 


F-1-3 


0.646 


5.802 


N/A 


N/A 


0.632 


5.927 


0.672 


5.680 


0.653 


6.473 


0.651 


5.337 


0.620 6.289 


N/A 


N/A 


F-1-4 


0.644 


6.093 


N/A 


N/A 


0.639 


5.682 


0.631 


5.401 


0.617 


6.312 


N/A 


N/A 


N/A N/A 


N/A 


N/A 


F-2-1 


0.652 


6.799 


0.666 


6.280 


0.678 


5.895 


0.675 


6. 101 


0.661 


6.167 


0.651 


5.921 


0.624 6.068 


0.648*6.270 


F-2-2 


N/A 


N/A 


0.661 


6.256 


0.646 


6.676 


0.684 


6.084 


0.653 


6.566 


0.664 


6.528 


0.650 6.820 


0.662*6.496 


F-2-3 


0.668 


6.068 


N/A 


N/A 


0.656 


6.081 


0.687 


6. 100 


0.665 


6. 143 


0.664 


6.376 


0.650 6.319 


N/A 


N/A 


F-2-4 


0.653 


6.141 


N/A 


N/A 


0.652 


6.171 


0.658 


6.047 


0.626 


5.746 


N/A 


N/A 


N/A N/A 


N/A 


N/A 


F-3-1 


0.679 


5.350 


0.674 


4.936 


0.651 


5.916 


0.663 


6.058 


0.657 


5.479 


0.653 


5.394 


0.668 5.570 


0.666 


5.405 


F-3-2 


N/A 


N/A 


0.660 


4.967 


0.627 


5.922 


0.642 


6. 172 


0.655 


5.826 


0.646 


5.388 


0.652 5.684 


0.653 


5.530 


F-4-1 


0.657 


5.860 


0.631 


5.337 


0.651 


5.769 


0.637 


5.383 


0.655 


5.798 


N/A 


N/A 


0.651 5.370 


0.648 


5.354 


F-4-2 


N/A 


N/A 


0.623 


5.655 


0.627 


5.740 


0.616 


5.451 


0.634 


6.651 


N/A 


N/A 


0.622 5. 136 


0.690 


7.331 


C-1-1 


0.572 


5.905 


0.548 


5.322 


N/A 


N/A 


0.563 


5.438 


0.584 


5.025 


0.567 


5.672 


0.548 5.276 


0.528 


5.877 


G-1-2 


N/A 


N/A 


0.567 


5.429 


N/A 


N/A 


0.568 


5.431 


0.566 


5.010 


0.565 


5.066 


0.514 5.305 


0.537 


5.899 


G-1-3 


0.553 


5.480 


N/A 


N/A 


0.539 


5.347 


0.581 


5.039 


0.552 


5.538 


0.546 


4.550 


N/A N/A 


N/A 


N/A 


G-1-4 


0.563 


5.463 


N/A 


N/A 


0.549 


4.948 


0.503 


4.933 


0.538 


5.884 


N/A 


N/A 


N/A N/A 


N/A 


N/A 


G-2-1 


0.550 


5.540 


0.556 


5.253 


0.553 


5.238 


0.559 


5.168 


0.560 


5.455 


0.560 


5.928 


0.516 5.325 


N/A 


N/A 


G-2-2 


N/A 


N/A 


0.534 


5.201 


0.541 


5. no 


0.514 


4.590 


0.541 


5.543 


0.547 


5.638 


N/A N/A 


N/A 


N/A 


G-2-3 


0.546 


5.397 


N/A 


N/A 


0.566 


5.052 


0.587 


4.968 


0.563 


5.539 


0.564 


5.458 


0.521 5.578 


N/A 


N/A 


G-2-4 


0.529 


5.367 


N/A 


N/A 


0.569 


5.528 


0.563 


4.824 


0.555 


5.645 


N/A 


N/A 


N/A N/A 


N/A 


N/A 


C-3-1 


0.586 


5.329 


0.548 


4.614 


0.557 


5.281 


0.573 


5.241 


0.572 


4.906 


0.569 


5.599 


0.572 4.974 






C-3-2 


N/A 


N/A 


0.549 


4.550 


0.559 


5.604 


0.576 


5.587 


0.570 


4.997 


0.572 


5.723 


0.582 5.139 






G-lt-1 


U.588 


5.587 


0.601 


5.865 


0.572 


5.258 


0.569 


5.429 


0.571 


4.952 


N/A 


N/A 


0.550 4.438 


0.558 


5.204 


C-4-2 


N/A 


N/A 


0.595 


5.894 


0.557 


5.002 


0.553 


5.281 


0.559 


4.975 


N/A 


N/A 


0.539 4.824 


0.554 


5.429 


H-l-1 


0.636 


5.776 


0.615 


5.672 


0.627 


5.380 


0.613 


4.907 


0.656 


5.230 


0.633 


4.790 


0.608 5.522 


0.594 


5.286 


H-l-2 


N/A 


N/A 


0.641 


5.483 


0.618 


5.338 


0.638 


5. 177 


0.648 


5.068 


0.638 


4.590 


0.594 5.423 


0.581 


5.229 


H- 1-3 


0.632 


5.770 


N/A 


N/A 


0.624 


5. 180 


0.654 


5.455 


0.643 


5.990 


0.627 


5.821 


0.623 5.575 


N/A 


N/A 


H-1-4 


0.643 


5.505 


N/A 


N/A 


0.617 


4.405 


0.628 


4.901 


0.584 


5.444 


N/A 


N/A 


N/A N/A 


N/A 


N/A 


H-2-1 


0.625 


6.014 


0.618 


5.654 


0.630 


5.902 


0.638 


5.575 


0.618 


5.540 


0.615 


5.350 


0.518 5.851 


0.575" 


5.096 


H-2-2 


N/A 


N/A 


0.629 


5.710 


0.626 


5.887 


0.632 


5.295 


0.624 


5.620 


0.629 


5.490 


0.62O 5.680 


0.534« 


4.356 


11-2-3 


0.623 


6. 108 


N/A 


N/A 


0.659 


6.344 


0.663 


5 . 904 


0.624 


5. 790 


0.643 


5.670 


0.628 5.208 


N/A 


N/A 


H-2-4 


0.607 


5.930 


N/A 


N/A 


0.637 


6.049 


0.622 


5.258 


0.597 


5.677 


N/A 


N/A 


N/A N/A 


N/A 


N/A 


H-3-1 


0.638 


5.620 


0.647 


5.590 


0.665 


5.630 


0.634 


5.29 3 


0.629 


5.440 


0.638 


5.040 


0.628 5.510 


0.616 


4.941 


H-3-2 


N/A 


N/A 


0.627 


5.571 


0.653 


5.450 


0.639 


5.599 


0.630 


5.650 


0.639 


5.010 


0.645 5.740 


0.636 


5.117 


H-l|-1 


0.640 


5. 144 


0.648 


5.218 


0.635 


5. 190 


0.652 


6.212 


0.585 


5.655 


0.605 


4.072 


0.593*4.769 


0.583 


4.467 


H-4-2 


N/A 


N/A 


0.653 


5.320 


0.653 


5. 116 


0.643 


5.678 


0.623 


5.309 


0.633 


4.464 


0.503*4.993 


0.620 


5.072 



INDICATES THAT RETEST WAS PERFORMED AT EXPOSURE SOMEWHAT OIFFERENT FROM THAT SHOWN. 

)TE3: 1) ALL COEFFICIENTS BASED ON COLLECTOR CROSS AREA. 
2) UNITS OF FRUL ARE DECREES! C ) /W/SQ H . 



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somewhat. This general trend is observed in more than one-half of the results shown. Attempts to 
correlate the 30 - 60 exposure day peak with particular collector materials were not successful. 
One possible factor, reported in recent literature [41], is that the transmittance values of poly- 
mer covers exhibit an initial increase followed by a monotonic decrease with exposure. However, 
the peak was observed for collectors with glass as well as polymer covers. It is also possible 
that the peak behavior could occur as a result of two or more compensating mechanisms [42]. It 
appears that further examination of the 30 - 60 exposure day peak behavior may be warranted since 
the mechanisms responsible for this pattern may be significant in investigating the long-term 
durability of collector materials. 

Considering the series 3 and 4 tests, the intercept peaks are more distinct at 30 to 60 days except 
for the collectors G and H. Generally, the series 4 exposure procedure, which was discontinued 
after 60 days, appears to be much more severe than the other three series. Although there are many 
exceptions, the results obtained in series 4 after 60 days are approximately equal to the results 
obtained in the other three test series after 480 days. This observation suggests the possibility 
of designing accelerated exposure tests based on concentrated solar radiation. 

Of the four test sites, the collectors exposed at site 1 show apparent degradation most 
frequently. As previously noted, this site is in a desert environment with a relatively high 
ambient temperature and with clear sky conditions common. This combination of environmental 
conditions results in higher collector stagnation temperatures than collectors tested at the other 
four sites. On the other hand, the cumulative solar irradiation received by collectors at this 
site in 480 days with a minimum of 17,000 kJ/m -day is less than the cumulative total received at 
the other sites. The reason is that this laboratory receives amounts of solar irradiation in 
excess of the qualifying 17,000 kJ/m -day nearly every day. These facts suggest that stagnation 
temperature may be the key parameter in inducing accelerated degradation. 

The reported results from site 4 had greater variability than the results obtained at the other 
three sites. It is not apparent from the information available whether this trend resulted from 
differences in climatic conditions or test procedures. 

The series 4 tests also provide further evidence that elevated stagnation temperature may give 
accelerated degradation. In series 4 tests, augmented exposure through the use of mirrors showed 
the most consistent pattern of performance degradation through the first 60 exposure days. The use 
of reflectors resulted in levels of solar irradiaton higher than would otherwise be experienced. 
As a consequence of augmentation, higher stagnation temperatures were achieved in collectors 
undergoing this series of tests. Unfortunately, difficulties in achieving uniform solar irradiation 
on the collectors arose from the use of reflectors. This problem along with other experimental 
difficulties resulted in the cancellation of this test series after 60 exposure days. 

4.3 ANALYSIS OF SLOPE AND INTERCEPT DATA 

As noted in the previous section, it is difficult to find obvious trends among the data from the 
test program. The measured values of F (xa) and F R U T exhibit considerable scatter and it does not 
appear justified to present and analyze all the collector results in depth. Therefore, collectors 
with typical results will be selected to analyze slope and intercept data statistically. 

Figures 4.3.1 through 4.3.3 show typical efficiency curves for three collectors. Collector B is an 
extremely stable collector and the 0-day through 480-day efficiencies show a normal spread but no 
evidence of materials degradation. Collector E shows significant apparent degradation in thermal 
performance after an initial exposure period of approximately 30 days. Figure 4.3.2 shows that the 
30-day retest curve represents the highest overall collector efficiency and that thermal 
performance decreased for subsequent exposure. (The reason that the 0-day curve is the lowest is 
unknown.) The results for collector F are shown in figure 4.3.3. A similar decline in thermal 
performance is evident in the figure. The absorber samples tested for this collector exhibited 
consistent degradation in absorptance and a slight improvement in emittance as shown elsewhere in 
the report. Correlations between materials sample degradation and collector efficiency results are 
analyzed in a later section. 

The results in the previous section showed that generally changes in the two primary collector 
performance parameters were of about the same order of magnitude as the nearly random scatter in 
measured results. Consequently, it is in order to investigate further the accuracy associated with 



o . 
co 



r\i_ 



COLLECTOR B 
SITE 1 SERIES 





- 


DRY 


+ 


3 - 


DOT 


X 


30 - 


DRY 


— — « 


60 - 


DAT 


* 


120 - 


DRY 


X 


240 - 


DRY 


- z 


480 - 


DRY 




0.00000 



0.08000 



TI-TR/G CC/N/SQ H) 



Figure 4.3.1. Collector B aggregate plot of measured efficiency, series 1, test site 1, all retests 



CD 



CM 



COLLECTOR E 
SITE 1 SERIES 1 



A 


- 


- DRY 


+ 


3 - 


- DRY 


X 


30 - 


- DRY 


♦ 


60 - 


- DRY 


♦ 


120 - 


- DRY 


X 


240 - 


- DRY 


z 


480 - 


- DRY 




0.00000 



0.08000 



TI-TR/G (C/W/S0 M: 



Figure 4.3.2 Collector E aggregate plot of measured efficiency, series 1, test site 1, all retests 



86 



CO 






^ 



COLLECTOR F 
SITE 1 SERIES 





- 

3 - 


DOT 
DRY 


X 


30 - 


DAY 


o 


60 - 


DRY 


■* 


120 - 


DRY 


X 


2140 - 


DRY 


— _ z 


1480 - 


DRY 




0.00000 



0.08000 



TI-TR/G (C/W/SQ Ml 



Figure 4.3.3 Collector F aggregate plot of measured efficiency, series 1, test site 1, all retests 



87 



the experimental facilities, variations in measured results within and between test sites, and the 
effects of environmental factors. 

4.3.1 Variation Within and Between Test Sites 

Variations in the reported values of the collector parameters result from experimental error in 
measuring collector energy output as well as from differences in collector material parameters. In 
order to emphasize differences associated with experimentation and test specimens of the same col- 
lector, results for an extremely stable collector (B) are investigated initially. Measurements of 
both collector efficiency and material properties, before and during exposure, showed that the 
performance of collector B was unchanged during the entire test program. 

Figures 4.3.4 and 4.3.5 show typical variations of slope and intercept values within a given site for 
all four test series. The apparent peaking of F (xa) and other characteristics of these measured 
parameters follow the same general trends as noted earlier. Figures 4.3.6 and 4.3.7 show the primary 
collector performance parameters plotted versus time for test series 1 at all test sites. The results 
on these two plots are typical of between-site differences. As expected, the results are less con- 
sistent between sites with correspondingly higher standard deviations. Within a given test site, the 
standard deviation is about the same as estimated measurement uncertainty [24,25]. For a given 
collector and test series, the calculated standard deviations are higher for the results which 
include all four test sites. These two observations apply to stable collectors such as collector B. 

Table 4.3.1 shows the results of a statistical analysis of the initial and final values of the two 
performance parameters. The results shown are based on test series 1 and 2 for each collector as 
reported by all four test sites. For series 2, the 3-day tests are used as the initial results. The 
final data are the 480-day results or the last test in those cases where the exposure period was 
shortened somewhat because of time limitations. Only collectors G and H, and possibly D, show a 
definite decrease in the mean value of the efficiency curve intercept. On the other hand, the loss 
coefficient parameter, F U generally shows a slight decrease for most of the collectors. The 
coefficient of variation for the intercept parameter generally increased for the collectors after 
exposure, but no clear trend is shown for the slope parameter variation. It should be noted that the 
above comparison includes unequal aging effects for the same model collector at different test sites. 
An increase in the coefficient of variation for the intercept parameter can be interpreted as meaning 
that the collector performance changes in a different manner at individual sites. Changes in the 
slope parameter, and to a lesser extent, the intercept parameter are undoubtedly masked by differences 
in test procedures, apparatus, and prevailing climatic conditions. 

Table 4.3.2 shows that the scatter of measured efficiencies around the linear correlating curve was 
less for the final test as compared to the initial test for a typical stable collector. The table 
shows that the residual standard deviation decreased approximately 50 per cent for collector B under 
the series 1 test procedure. (The values of F (tcO and F R U in table 4.3.2 included all reported 
data. These values may be somewhat different from those in table 4.2.1 based on results reported 
by the test laboratories who often used a subset of all efficiency values reported to generate an 
efficiency curve.) The results shown in table 4.3.2 indicate that the test laboratories improved 
their procedures with repetition and experience. 

4.3.2 Influence of Experimental Apparatus 

Generally, changes in the two primary performance parameters were of about the same order of magnitude 
as the random scatter in measured results for unexposed collectors. Consequently, the accuracy 
associated with the experimental facilities and possible site bias should be investigated. 

There are no obvious degradation trends with exposure time observed in much of the reported data. 
Increases in Fr(tci) and decreases in F^Ut were frequently observed. Several collectors, especially 
those tested at site 3, experienced a consistent increase in F^(xa) or a decrease in F^U^ throughout 
the entire exposure period. In other words, based on the reported results, after nearly three years 
of exposure the performance of several collectors was apparently better than the original performance. 
Physical reasoning dictates that such results be viewed with suspicion. Because of these anomalies, 
the experimental apparatus and procedures were investigated. An analysis of the effect of changes in 
pyranometer sensitivity with time was carried out. The temperature dependence of pyranometer perfor- 
mance was also examined. Finally, correlations were performed to determine whether excessive data 
scatter or suspicious results were associated more with particular pyranometers or test stands. 



88 



5 i. 

C o 

b 



ID CODE 

OB-1-1 
AB-1-2 
+ B-1-3 
XB-l-H 



RESULTS RS REPORTED 



UNCERTRINTY. ± 0.01 
MERN VRLUE. 0.66 
STD DEVIRTION.0.01 



~l 1 1 1 - 

3 IS 30 60 

EXPOSURE TIME ( DAYS ) 



— I - 
120 



— I 

480 



240 



Figure 4.3.4 Collector B efficiency curve intercepts vs exposure time, test site 1, all series 



O 



°.-\ 



RESULTS RS REPORTED 



3 o 






o 



ID CODE 

OB-1-1 
AB-1-2 
+ 8-1-3 
XB-1-4 



UNCERTRINTY. ± 0.20 
MERN VRLUE. 5.36 
STD DEVIATION. 0.20 



-, , , ,_ 

3 IS 30 60 
EXPOSURE TIME ( DRYS ) 



— r~ 

120 



240 



480 



Figure 4.3.5 Collector B efficiency curve slope parameter vs exposure time, test site 1, all series 



89 



i » 

a o' 

i ■» 

a o" 

fc 



RESULTS AS REPORTED 



ID CODE 

OB-1-1 
AB-2-1 

+ B-3-1 
XB-1-1 



UNCERTAINTY. ± 0.01 
MEAN VALUE. 0.67 
STD DEVIRTION.0.02 



1 
15 
EXPOSURE 



1 r 

30 60 

TIME ( DAYS ) 



120 



240 



180 



Figure 4.3.6. Collector B efficiency curve intercepts vs exposure time, all test sites, series 1 



a 





















o 








RESULTS 


AS REPORTED 








o 

00 


















o 
r-~ 

o 
u>~ 


















*. 


X 


* 

X 

+ 


X 


., J-.... 


A 
© 


* 


© 


o 


s 






6 


© 

X 


+ 
X 


£ 
o 


+ 


o 

sr~ 


















o 


ID CODE 
















en 


OB-1-1 
ab-2-1 
+ B-3-1 








UNCERTRINTY. * 0.20 
MEAN VALUE. 5.35 
STD DEVIATION.0.91 






o 






ru 






o 


XB-M-1 








O 








O 


















o 


1 



I 
3 


1 
15 


i 

30 


1 
60 


1 

120 


1 

240 


1 
480 



EXPOSURE TIME (DAYS ) 

Figure 4.3.7. Collector B efficiency curve slope parameter vs exposure time, all test sites, series 1 



90 



Table 4.3.1. Statistical Analysis of Efficiency Curve Intercept and Slope Parameters 
Based on Initial and Final Tests, All Collectors and Test Sites, Series 
1 and 2 







Intercept F (ret) 
R 




-Slope 


C W 










Std Error of mean 






Std Error of mean 








Coef of 
Variation 








Coef of 
Variation 






Collector 


Mean 


Value 


Per cent 


Mean 


Value 


Per cent 


A 


i* 


0.615 


3.034 


0.00659 


1.073 


4.575 


13.640 


0.2206 


4.823 




f* 


0.620 


4.185 


0.00917 


1.480 


4.225 


8.994 


0.1344 


3.180 


B 


i 


0.654 


2.302 


0.00532 


0.814 


5.499 


5.911 


0.1149 


2.090 




f 


0.674 


2.184 


0.00521 


0.772 


5.285 


3.784 


0.0707 


1.338 


C 


i 


0.533 


3.174 


0.00598 


1.122 


4.204 


4.459 


0.0663 


1.576 




f 


0.513 


5.461 


0.01060 


2.064 


3.677 


10.950 


0.1522 


4.139 


D 


i 


0.646 


2.006 


0.00458 


0.709 


3.213 


10.626 


0.1207 


3.757 




f 


0.639 


2.495 


0.00563 


0.882 


3.150 


8.302 


0.0924 


2.935 


E 


i 


0.607 


2.094 


0.00449 


0.740 


6.418 


8.123 


0.1843 


2.872 




f 


0.611 


5.328 


0.01151 


1.884 


6.189 


8.733 


0.1911 


3.087 


F 


i 


0.654 


2.621 


0.00606 


0.927 


6.015 


11.089 


0.2358 


3.921 




f 


0.652 


3.264 


0.00753 


1.154 


6.127 


10.997 


0.2382 


3.888 


G 


i 


0.568 


3.831 


0.00769 


1.354 


5.429 


7.982 


0.1532 


2.822 




f** 


0.544 


2.600 


0.00708 


1.300 


5.602 


6.116 


0.1713 


3.058 


H 


i 


0.636 


1.437 


0.00323 


0.508 


5.580 


4.849 


0.0957 


1.714 




f 


0.592 


5.397 


0.01130 


1.908 


4.946 


7.008 


0.1225 


2.478 



* i denotes initial test: 0-day for series 1, 3-day for series 2 

f denotes final test : 480-day or somewhat less 
** based on 4 data sets available 



Table 4.3.2. Residual Standard Deviations for Collector 
B Test Series 1 Linear Efficiency Curve 
Correlations 



Site 


Test 
id 


No. of 
data pairs 


Intercept 
(percent) 


C-) Slope 
CW/m 2 -°C) 


Residual std 
deviation 


1 


0-day 


16 


65.3 


5.46 


0.992 


480-day 


16 


65.9 


5.49 


0.544 


2 


0-day 


16 


65.5 


5.49 


1.904 


480-day 


16 


68.3 


5.47 


1.092 


3 


0-day 


16 


66.2 


5.61 


0.544 


480-day 


16 


70.0 


5.46 


0.659 


4 


3 -day 


12 


69.9 


5.86 


1.107 


480-day 


40 


67.3 


5.01 


0.630 



91 



The investigation of possible interlab differences in reported results was conducted to determine 
whether any test site consistently reported values of efficiency curve intercepts or slope which 
were higher or lower than the mean of the results from the other laboratories. The approach taken 
was to plot the values of the parameters from each of the four sites on a common graph vs exposure 
using series 1 and 2 results. Table 4.3.3 shows the results as the percentage of times that a 



particular test site reported extreme values of F (xa) 



The table shows the times that a particular 



Table 4.3.3. Comparison of Frequency of Extreme Values 

for F (xa) Reported by Test Sites 

R 



Site 


Percentage of times a site reported values: 


highest 


lowest 


highest or lowest 


1 


10.9 


32.9 


21.5 


2 


14.7 


36.3 


25.5 


3 


43.9 


7.9 


25.9 


4 


30.5 


23.7 


27.1 



site reported either the highest, lowest, or highest or lowest values. The latter is a measure of 
the excess spread in reported results. 

As shown in the table, sites 1 and 2 reported the lowest readings most often while sites 3 and 
4 reported the highest readings more often. Caution is necessary in interpreting these results. 
The tabulation considers only the relative magnitudes of the reported values, i.e., the highest and 
lowest readings were tabulated but the quantitative amount by which a reading was the highest or 
lowest is not reflected in the tabulations. Also, the results presented may be affected by pre- 
vailing climatic conditions. The effects of environmental conditions on reported results are con- 
sidered in a later section. Based on the last column in table 4.3.3, extreme readings, on the 
average, were essentially evenly distributed between the four test sites. This observation suggests 
relative consistency between the sites. 

An analysis was made to determine if a particular pyranometer or test stand within a test site could 
be identified as a major source of the scatter in data. This evaluation was carried out by 
identifying values of F (xa) which fell outside the calculated experimental uncertainty limits as 
measured from linear curve fits of F (xa) as a function of exposure time for a particular collector 
- series combination. The experimental uncertainty in determining F (xa) resulting from instru- 
mentation tolerances was taken to be 0.01 as calculated from a Kline-McClintock type analysis by 
Culkin [43]. Any values of F (xa) beyond this uncertainty from the linear fit were tabulated along 
with the pyranometer serial number and test stand number within the test site used for that result. 
The number of times each pyranometer and test stand were used was also tabulated in order to prevent 
an unjust bias of the data. For data falling outside the measurement uncertainty band, the per- 
centage of times a particular pyranometer or test stand was used was then tabulated. Table 4.3.4 
shows the results of this analysis through 120 exposure days. 

The tabulated results show that no one pyranometer or test stand can be identified as a primary 
source of suspect data. It should be noted that of the points identified outside the permissible 
band, as many were found to be high as were found to be low. This results also held true for 
individual pyranometers . In other words, the points identified as suspect were randomly distributed. 
The pyranometers and test stands yielding such data were random as well. 

An analysis was performed to examine the effect of pyranometer sensitivity changes on the reported 

results. From pyranometer calibration histories provided by two of the participating laboratories 

and the pyranometer manufacturers, it was observed that pyranometer sensitivity generally decreases 

with time and increasing ambient temperature. The net effect of a pyranometer sensitivity decrease 

is to charge the collector with less solar irradiance than it actually receives. Consequently, 

measured values of F (xa) would be greater than actual. On the other hand, pyranometer sensitivity 
R 



92 



changes with time have no effect on measured values of F U provided such changes are negligible 
during a given test. 

The reported values of solar irradiance in the plane of the collector must be corrected in order to 
compensate for changes in pyranometer sensitivities. Generally, the pyranometers used in the test 
program were calibrated on an average of two times per year rather than at the time of each retest. 
Therefore, a method of interpolating calibration results was necessary to obtain an approximation 
for the sensitivity at the time of the test. A linear interpolation procedure was used. 



Table 4.3.4. Correlations of Pyranometers and Test 

Stands with Experimental Results Outside 
Probable Measurement Uncertainty Ranges, 
Site 1, All Series through 120-day Retests 



Pyranometer 
Serial No. 
or Test 
Stand 


Times 
Used 


Percentage of times used 
When values beyond 
Uncertainty Range 


14317 


16 


18.75 




14318 


25 


12.00 




14319 


13 


38.46 




14320 


5 


40.00 




14321 


21 


9.52 




14322 


10 


30.00 




17349 


25 


20.00 




18273 


14 


42.86 




Stand 1 


12 


25.00 




Stand 2 


15 


26.67 




Stand 3 


25 


16.00 




Stand 4 


23 


13.04 




Stand 5 


24 


25.00 




Stand 6 


32 


31.25 





The reported values of F (xa) from site 2 were particularly appropriate for examining the effects of 
pyranometer sensitivity changes. This site used pyranometer calibration constants from calibrations 
performed at the beginning of the test program for data reduction throughout. The other partici- 
pating laboratories generally used updated calibration constants from periodic recalibrations . 
Consequently, the values of the intercept parameter reported by site 2 represent a limiting worst 
case with regard to possible errors caused by pyranometer sensitivity changes. 

Typical pyranometer sensitivity curves are presented in figure 4.3.8. Generally, maximum changes of 
from two to three percent in the reported values of F (xa) resulted from pyranometer sensitivity 
corrections. This correction had no significant effect on reducing the data scatter in the reported 
results. While the sensitivity changes introduced a minor consistent error in the reported results, 
this source of error was found to be too small to mask any significant degradation trends . After 
correction, the values of F (xa) still exhibited random fluctuations of at least the same order of 
magnitude as any general degradation trend. 

A correction methodology was also applied to account for instrument sensitivity to ambient temper- 
ature. For a given solar irradiance, the voltage signal output of pyranometers decreases with 
increasing ambient temperature. The instrument sensitivity to operating temperature was assumed to 
be invariant with time and taken from the manufacturer's calibration information. It was found that 
temperature compensation did not result in any significant changes in the reported results. 
Generally, the effect of temperature dependence on pyranometer output was insignificant. 



93 



o 

rvi 



in 



X - 

az 
o 



<n 



o 

u 



—i 
u 



Q 



m 

O) . 



CALIBRATION CURVE FOR 
PTRANOMETER 1W321 



.40 




-10 



— T" 
20 



50 



CALIBRATION CURVE FOR 
PTRflNOMETER 14322 




-40 -10 



20 



CALIBRATION CURVE FOR 
PTRRNOMETER 14392 




Q 
cm 



az 

o 



m 

o - 



CO 

z 
o 



cr a 

O 01 



a 

Id 
fvi 



en 

2 
O 



u 



— ' ' I — '— ' — I — ' — ' — I 

-40 -10 20 SO 

MONTHS (FROM JAN. 



CflLIBRflTION CURVE FOR 
PTRflNOMETER 17349 



1979) 



-40 



-10 



20 



I 

50 



in 



o 
o . 






CflLIBRflTION CURVE FOR 
PTRflNOMETER 19241 




— 






— 


CflLIBRflTION CURVE FOR 
PTRflNOMETER 15839 


o 
o- 




9— a 


— 




V 


in 
o 




\ 


a 






a. 


— "— 


-1 | 1 , 1 r 1 



-40 -10 
MONTHS 



20 



50 



(FROM JAN. 1973! 



m 

o- 



m 

OJ- 



CflLIBRflTION CURVE FOR 
PTRflNOMETER 18273 



O CC 

VS M0NUM 




-40 



•10 



— T" 
20 



— 1 
50 






in 

01 - 



CALIBRATION CURVE FOR 
PTRflNOMETER VPI. (REr.) 



T— i ' 1 ' ' 1 ' ' 1 

-40 -10 20 SO 

MONTHS (FROM JAN. 1979) 



-r 



■to 



-40 

MONTHS 



— i - 
20 



50 



(FROM JRN. 1979) 



Figure 4.3.8. Typical Pyranometer calibrations vs time 



94 



4.3.3 Environmental Factors 

The measurement of collector efficiency, which is the basis for investigating the thermal 
performance of collectors in the present study, may be affected by variations in test environments 
within and between the four test sites. The results of calculations to determine the expected 
effect on measured efficiency of different environmental parameters, such as wind speed, ambient 
temperature, total and scattered solar irradiance, and beam incident angle, are presented in this 
section. Such parameters can substantially affect both the heat transfer and optical 
characteristics of various collectors. 

The parameter F„U-r is much more sensitive to variations in the test environment than is F (xa). The 
overall loss coefficient IL represents the sum of the top, edge, and back loss conductions per unit 
aperture area. For well designed collectors including those used in the test program, most heat 
loss occurs through the top cover assembly. The top loss coefficient is a function of the 
convective heat transfer coefficient which, in turn, depends on wind speed, ambient temperature, and 
mean absorber plate temperature. The top loss coefficient also depends indirectly on the total and 
scattered solar irradiance since these parameters affect the mean absorber temperature. 
Consequently, U, and F R U T depend on the prevailing environmental conditions at the time of each 
efficiency test. 

Since variations in the test environment were suspected to cause some of the random variations noted 
in the two collector parameters, an analytical procedure was used to adjust the results of 
efficiency tests under any particular set of test conditions. The motivation for correlating test 
results to a standard set of conditions was twofold. First, the spread in reported values 
of F (m) and F„Ut which may be attributed to different environmental conditions should be 
reduced. Secondly, adjustment to a standard set of test conditions provides a common basis to 
compare results for the same model collector at different test sites. If the measured results 
correlate with test conditions, a correction of this type is necessary to determine whether 
interlaboratory differences in the testing procedures occurred. 

An analytical procedure was developed earlier to effect correction of test data to a standard test 
condition. A detailed description of the correction procedure and mathematical model used is presented 
in Reference [40]. The correction is based on using an analytical model of the collector to calculate 
efficiencies at standard and at actual test conditions for a particular [t^ - t a ]/G. The difference 
between these calculated efficiencies is then added to the measured efficiency, i.e., 

n(X, std) = n (X, test) + [n , (X, std) - n , (X, test)] 
meas calc caic 

where 

X = [t. - t ]/G 
1 a 

is held constant. As shown in Reference [40] , the experimental verification of this procedure has 
been reasonably successful provided the collector configuration fits the analytical model used. 

The information required to apply the correction methodology includes pertinent weather parameters, 
operating conditions during testing, and physical dimensions of the collector. In addition, heat 
transfer and optical characteristics of the various collector components are necessary. The 
standard test environment used for the present investigation is shown in table 4.3.5. The values 
shown in this table represent approximately median weather conditions of the test conditions 
experienced throughout the test program. 

The correction procedure as outlined was used on the reported test results for collectors B and E. 
These collectors were selected for analysis because they are typical of the most prevalent designs 
and employ construction materials that are commercially available. Both collectors have parallel 
flow through the absorber which is easier to model than the serpentine case. Collector E has a 
single FRP cover with a flat-black painted copper absorber. Collector B has a double, low-iron, 
glass cover assembly with a flat-black copper absorber. The double-covered collector was expected 
to be influenced less by variations in environmental conditions. The physical characteristics and 
heat transfer properties used in the correction were based on measurements obtained at the time of 
initial exposure. These values are tabulated in Reference [25]. 



95 



Table 4,3.5. Reference Environment Used in Compensation 
for Environmental Dependence of Reported 
Results 



Condition 


Value 


Ambient temperature 


20°C 


Solar: Irradiance 


1000 W/m 2 


Beam angle 


15 deg 


Diffuse fraction 


0.15 


Wind speed 


3.0 m/s 


Test fluid 


water 


Flow rate 


0.02 kg/s-m 2 


Collector slope 


45.0 deg 



Figure 4.3.9 shows a comparison between the calculated efficiency curve and an aggregate plot of all 
measured instantaneous efficiencies for collector B, series 1 at site 1 (B-l-1 0- through 480-day 
retests) . This figure shows the typical agreement between the model and data which is judged to be 
satisfactory for calculating efficiency differences attributed to environmental variations. The 
correction procedure was then carried out to determine whether a significant amount of the scatter 
in the reported values of F (to) and F U could be attributed to variations in environmental factors. 
Plots of the "corrected" values vs exposure time were prepared and comparisons of these were made to 
the plots of the "as reported" results. Figures 4.3.10 through 4.3.21, typical plots of this type, 
show that compensation for environmental differences at the time and location of the test does 
not significantly alter the reported results. 

An analysis of the graphical results shows that no detectable changes in trends, or lack of trends, 
in the values of F (xa) and F U resulted from taking into account different environmental test 
conditions. The mean values of F (xa) generally remained within 0.01 efficiency per cent of the 
mean values of the reported results by individual sites or by series. The mean values of F U 
actually increased somewhat. This slight increase probably resulted from the standard value of wind 
speed being somewhat higher than the average wind speed experienced during testing. More impor- 
tantly, the standard deviations about the mean values were not reduced significantly for either F U 
or F (xa) . In fact, the standard deviation in F (xa) increased as a result of the analytical 
correction procedure. This unexpected result is attributed to a lack of responsiveness or precision 
in the test procedure. As shown in the tabulations of thermal performance data in appendix C of 
Reference [25], the measured efficiency trends do not always follow those expected when environmental 
parameters change. For example, there are cases where the wind speed increases somewhat while all 
the other operating conditions remain constant; however, the measured efficiency does not always 
decrease in these cases. It appears that the time response of the collectors along with inherent 
experimental uncertainties associated with the test method may be contributing to this anomaly . 

While the plots of the corrected values of F (xa) and F U revealed no new trends, the weather 
compensation procedure did not affect the observed 30 - 60 exposure day peak behavior in F (xa) 
discussed earlier. This behavior, therefore, does not appear to be caused by environmental changes 
during the testing. 

The analysis of variations in measured results caused by variations in environmental conditions show 
that such changes have no significant effect on the two primary efficiency curve parameters. 



96 



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ro 




o 




OJ 



LEGEND 

o 0-DF1 T x 60-DflY 

a 3-DHY o 120-DAY 

15-DAY ^-240-DAY 

+30-DAY XMOO-DRY 

zMODEL 




1 

0. 10000 



0.01000 



(TI-TR)/HT (C/H/M**2) 



Figure 4.3.9. Comparison of calculated and measured efficiencies for collector 
test series 1, all retest data 



site 1, 



97 



o 
a-, 

o 
en' 

o 

CD" 



~ O 

U 

. w 

E °. 

O in' 

3 o 



ID CODE 

OB-4-I 



RESULTS RS REPORTED 



T 
15 



UNCERTAINTY. * 0.20 
MEAN VfiLUE. 5.29 
STD DEVIATIONS. >18 



- r~ 

30 



1^ 
60 



120 



240 



180 



EXPOSURE TIME ( DAYS ) 

Figure A. 3. 10. Reported values of F U for collector B, site 4, test series 1 vs exposure time 



U 



ADJUSTED TO STANDARD 



ID CODE 
OB-M-1 



UNCERTAINTY, t 0.20 
MEAN VALUE. 5.37 
STD DEVIATIONS. 50 



1 

15 
EXPOSURE 



— i r 

30 60 

TIME ( DAYS ) 



— r - 

120 



240 



480 



Figure 4.3.11. Values of F U for collector B, site 4, test series 1 vs exposure time, adjusted to 
standard test conditions 



98 



a 

b 



RESULTS AS REPORTED 



ID CODE 

OB-1-1 



UNCERTAINTY. « 0.01 
MEAN VfiLUE. 0.68 
STD DEVIATION.0.03 



~i 1 1 r 

3 15 30 60 

EXPOSURE TIME ( Dfi-YS ) 



I 
120 



210 



180 



Figure 4.3.12. Reported values of F (ta) for collector B, site 4, test series 1 vs exposure time 



ADJUSTED TO STANDARD 



_© 



© 



S - 

a. o 



ID CODE 
OB-q-1 



UNCERTAINTY. * 0.10 
MEAN VALUE. 0.68 
STD DEVIATION. 0.02 



Figure 4.3 



1 1 1 v~ 

!5 30 60 120 

EXPOSURE TIME ( DAYS ) 



240 



480 



.13. Values of F„(,-) for collector B, site 4, test series 1 vs exposure time, adjusted 



to standard test conditions 



99 



RESULTS AS REPORTED 



10 CODE 

OB-l-1 
AB-2-1 
+ B-3-I 



UNCERTAINTY, i 0.20 
MERN VALUE. 5.37 
STD DEVIRTION.0.31 



-j i 1 r 

3 15 30 60 

EXPOSURE TIME ( ORYS I 



— r~ 

120 



— I — 
210 



— I 
180 



Figure 4.3.14. Reported values of F U for collector B, site 1, 2, and 3, test series 1 vs exposure time 



R L 



ADJUSTED TO STANDARD 



3 


o 


+ 


w 


zr 






o 


ID CODE 


L. 


cn~ 


OB-l-l 




o 


AB-2-1 




f\J 


+ B-3-I 



UNCERTRINTY. * 0.20 
MERN VRLUE. 5.M7 
STD DEVIATION. 0. 34 



~l 1 i 1 1 1 1 

3 15 30 60 120 210 180 

EXPOSURE TIME (DAYS I 



Figure 4.3.15. Values of F U for collector B, sites 1, 2, and 3, test series 1 vs exposure 
time, adjustea to standard conditions 



100 



RESULTS AS REPORTED 



o. 


in 




b 


a 




■ 


a* 






m 


ID CODE 




o 


OB-I-I 




c\i 


AB-2-1 




o 


■f B-3-1 



UNCERTAINTY. » 0.01 
MEAN VALUE. 0.66 
STD DEVIATIONS. 01 



~1 1 1 r - 

3 IS 30 60 
EXPOSURE TIME I DAYS » 



— T~ 
120 



— r~ 

210 



- 1 

180 



Figure 4.3.16. Reported values of F (ia) for collector B, sites 1, 2, and 3, test series 1 vs exposure time 



ADJUSTE0 TO STANDARD 



ID CODE 

OB-I-I 
AB-2-1 
+ B-3-I 



UNCERTAINTY. > 0.01 
MEAN VALUE. 0.66 
STD DEVIATIONS. 02 



"I 1 1 f~ 

3 IS 30 60 
EXPOSURE TIME ( DAYS I 



— I 

180 



I20 



I 

210 



Figure 4.3.17. Values of F (tcx) for collector B, sites 1, 2, and 3, test series 1 vs exposure 
time, adjusted to standard conditions 



101 






O m 
3 o 



RESULTS AS REPORTEO 



10 CODE 

OE-I-I 
AE-2-1 
+ £-3-1 



UNCERTAINTY. « 0.20 
HERN VALUE. 6.31 
STD DEVI AT I ON. 0.12 



1 1 r 

IS 30 60 

EXPOSURE TIME ( DAYS I 



- 1 — 
2-IO 



"~1 

180 



120 



Figure 4.3.18. Reported values of F U for collector E, sites 1, 2, and 3, test series 1 vs exposure time 



ID CODE 

© E-l-l 
a E-2-1 
+ E-31 



ADJUSTED TO STANDARD 



UNCERTAINTY. > 0.20 
MEAN VALUE. 6.63 
STD DEVIHTION.0.35 



1 1 1 1 1 r 1 

3 15 30 60 120 210 180 

EXPOSURE TIME ( OAYS I 



Figure 4.3.19. Values of F U for collector E, sites 1, 2, and 3, test series 1 vs exposure 
time, adjusted to standard conditions 



102 



ID CODE 

OE-I-I 
AE-2-1 
+ E-3-I 



RESULTS AS REPORTED 



UNCERTAINTY. > 0.01 

Hern value. o.6i 

STD DEVI AT I ON. 0.02 



! 1 1 - 

15 30 60 

EXPOSURE TIME I DAYS I 



~~ T~"~ 
210 



180 



120 



Figure 4.3.20. Reported values of F (tcc) for collector E, sites 1, 2, and 3, test series 1 
vs exposure time 



cJ1 



ADJUSTED TO STANDARD 



m 


ID CODE 


o 


O E-l-l 
A E-2-I 
+ E-3-I 



UNCERTAINTY. « 0.0I 
HE AN VALUE. 0.60 
STD DEVIATION. 0.02 



T 1 1 1- 

3 IS 30 60 
EXPOSURE TIME I DAYS I 



120 



— f— 
210 



~ 1 
160 



Figure A. 3. 21. 



Values of F (tcc) for collector E, sites 1, 2, and 3, test series 1 vs exposure time, 
adjusted to standard conditions 



103 



4.3.4 Effect on F da) of Linearized Efficiency Curves 
K 

The analysis of the reported values of the efficiency curve parameters is based on the assumption 
that efficiency can be adequately represented by a linear function of [t^ - t a ]/G. Actually, the 
elope of collector efficiency curves continually decreases with increasing temperature since the 
loss coefficient increases with absorber temperature. In this section, the validity of comparing 
values of F da) based on a linear efficiency curve as the characteristic parameter is established. 

A comparison is made of the values of F da) extrapolated from the linearized model with those 
obtained by instantaneous efficiency measurements where the inlet temperature was equal to the am- 
bient temperature. The "directly measured" values were taken to be all the instantaneous 
efficiencies obtained with [t. - t a ]/G less than or equal to 0.001°C/Wm 2 . The 

measured values are shown in figure 4.3.22 as the abscissa with the extrapolated values shown as 
the ordinate. Perfect agreement would correspond to the case where the points would lie on the 
solid line. The extrapolated values are slightly and consistently higher than those measured 
directly. Each extrapolated value, however, is within 1.0 efficiency per cent of the measured 
value. 

Based on figure 4.3.22, representing collector efficiency as a first-order function in [t. - t ] /G 
introduces a small but consistent error in the values of F (to) for collectors of the type used in 
the test program. Since this error is consistent and small, however, all efficiency curves may be 
assumed to be affected by about the same amount. Consequently, use of a linear efficiency 
correlation does not contribute to the scatter in reported values of F da) and has a negligible 
effect on the comparisons presented. 

4.4 COLLECTOR THERMAL PERFORMANCE DEPENDENCE ON MATERIAL PROPERTIES 

A mathematical model for collector thermal performance was used to calculate expected changes in 
efficiency curve parameters as a result of arbitrary and measured changes in several key material 
properties. The results are shown for changes in absorber plate emittance and absorptivity, cover 
normal beam transmittance, and conductivity of thermal insulation. The results calculated for 
typical changes in measured properties are then compared with measured changes of the collector 
efficiency curve parameters. 

Collectors D and H were selected for the theoretical investigation. The efficiency curve parameters 
and material properties for these two collectors showed significant changes after exposure. Both col- 
lectors could be mathematically modeled in a straight-forward manner. Collector D has two glass 
covers and a selective absorber. Tests on actual samples of the absorber plate showed that the emit- 
tance changed appreciably during the exposure period for some of the collectors exposed. Collector H 
has an outer cover of poly(ethylene terephthalate )(PET) and an inner fluorinated (ethylene propylene) 
copolymer (FEP) cover with a flat-black absorber. Tests showed that significant changes had occurred 
in the solar transmittance of the two cover system primarily due to outgassing deposits. 

The mathematical model used to calculate the thermal performance is based on the Hottel-Whillier- 
Bliss analysis with an extension to account for the serpentine flow configuration of collector H. 
A detailed description of the mathematical model is given in Reference [40], Table 4.4.1 shows the 
base case parameters and dimensions required by the analytical model for the two collectors. 
Tables 4.4.2 and 4.4.3 show calculated efficiencies for the base case and arbitrary changes in 
properties. The calculations shown are for changing only one property with others held at the base 
case value for a range of [tj - t ]/G. In order to compare with measured results, a linear curve 
was fit to the calculated efficiency values shown. The abscissa values were selected in accordance 
with ASHRAE 93-77 [6] as approximately 0.1, 0.3, 0.5, and 0.7 of [t ± - t a ]/G at stagnation 
conditions. The F da) and F IL values shown in the tables are then the intercept and negative 
slope of the correlating curves. The mean residual errors for the curve fit based on these four 
sets of values are also included in the table. In the tables, x , designates the solar beam 
transmittance of the inner cover (1) and outer cover (2). The results shown in the two tables are 
also depicted graphically in figure 4.4.1 where the parameters F da) and F R U. have been normalized 
by the case values. 

Several observations follow from examining the effect on F U da) and F R IL of changes in the four 
materials. As would be expected, changes by 0.01 W/m-°C in the thermal conductivity from the base 
case value have a strong effect on the slope parameter but a small effect on the intercept 
parameter. While conductivities were not measured before and after exposure, this property could 
change as a result of compaction, moisture entering fibrous insulation (Collector D) , thermal damage, 
or moisture entry into open pores of organic foam insulation (Collector H) . 



104 




AVERAGE DIFFERENCE « +0.617/100 



— . 1 1 , 1 , 1 , 1 1 , 

0.54 0.5S 0.58 0.60 0.62 0.64 0.66 



MEASURED 



Figure 4.3.22. Comparison of measured F (xa) to values extrapolated 
from linearized efficiency curves 



105 



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Table 4.4.1 Base Case Collector Parameters for Calculating 

Thermal Performance Dependence on Material Properties 



Dimension or Property 



Units 



Absorber 

Flow Configuration 
Effective Length 
Effective Width 
Flow Tubes: Number 

O.D. 

Hydraulic Diameter 

Wetted Perimeter 
Thickness 

Thermal Conductivity 
Emittance 
Solar Absorptance 

Cover Assembly 
Number of Covers 

Air Space: under cover 1/under cover 2 
Infrared Emittance: cover 1/cover 2 
Infrared Transmittance: cover 1/cover 
Index of Refraction: cover 1/cover 2 
Extinction Coefficient: cover 1/cover 
Thickness: cover 1/cover 2 

Insulation 
Thickness: Back 
Edge 
Conductivity: Back 
Edge 

Aperture Area 

Gross Area 



m 
m 

mm 
mm 
mm 
mm 
W/m°C 



Collector 



parallel 


serpe 


1.726 


2.337 


0.813 


1.130 


10 


8 


8.1 


15.88 


4.93 


12.70 


15.49 


39.90 


0.90 


1.778 


45.00 


200.0 


0.07 


0.89 


0.97 


0.96 



mm 


37.0/25.0 


25.4/12.7 


- 


0.84/0.84 


0.33/0.76 


2 


0.02/0.02 


0.60/0.12 


- 


1.30/1.30 


1.33/1.68 


2 mm 


0.0021/0.0021 


0.0275/0.1874 


mm 


3.18/3.18 


0.0254/0.1778 


mm 


88.9 


25.4 


mm 


25.4 


25.4 


W/mOC 


0.04 


0.02 


W/m°C 


0.04 


0.02 



1.39 



1.67 



2.64 



2.93 



An increase in the absorber plate emittance would be expected to increase the loss coefficient U^ 
and have an insignificant effect on the optical parameter (to) as shown since this property controls 
long wavelength radiation from the absorber. The increase in e p by 0.1 increases F R U L much more for 
collector D than for collector H since the former has a selective surface with a relatively low 
emittance value. Actual tests of collector D absorber plate samples showed in some cases that the 
plate emittance changed by approximately this amount in going from an initial value of 0.07 to 0.17 
after exposure. The emittance of test samples from collector H decreased slightly after exposure from 
0.89 and 0.86. 

A decrease of 0.1 in the solar absorptance of the plate decreases the intercept parameters of both 
collectors by about 8 per cent and has a negligible effect on the efficiency curve slopes. Tests on 
absorber samples from both collectors showed the solar absorptance was essentially unchanged from 
the base case value after exposure. 

The effect on F (xa) and F U of changes in the solar transmittance of the cover system is 
consistent with expectations. The decreases in transmittance decrease F (tot) proportionately but 
decrease F^U^ very little. The calculations are based on the assumption that the cover reflectance 
is unchanged; a decrease in transmittance is assumed to increase the cover absorptance by the same 
amount. Consequently, the covers are somewhat warmer than in the base case and slightly reduce heat 
loss from the absorber. While the glass cover material of collector D would not degrade with 
exposure, the antiref lective etching can lose its effectiveness, e.g., due to the buildup of outgassing 
condensation or dust. The cover system of collector H is more susceptible to degradation from 
exposure. Table 4.4.3 shows the effects of arbitrary decreases in solar beam transmittance of 0.10 
from the base case value. The last four columns show additional results calculated for actual 
measured changes in transmittance for the two covers. Two samples of the outer cover, an ultra- 
violet-resistant PET film, experienced a decrease in transmittance from 0.85 to 0.84 and 0.81. 



107 



Table 4.4.2. Effects of Material Property Changes on 



F D (ia) and F_U T for Collector D 
R R L 





base 


k + 0.01 


k - 0.01 


e + 
P 


10 


a - 0.10 
P 


x -. . - 0.10 
sbl 


x , - 0.10 
sb 




case 


(W/m-°C) 


(Wm-°C) 








(inner) 


(both) 


At/G 






Calculated efficiency (aperture area basis) 




(°C-m 2 /W) 








(%) 




0.02 


71.2 


69.3 


73.2 


70.3 




64.5 


66.0 


58.9 


0.05 


58.3 


55.1 


61.6 


56.6 




51.6 


53.3 


46.3 


0.08 


45.8 


41.3 


50.4 


43.0 




38.9 


40.7 


33.6 


0.12 


26.5 


20.2 


33.0 


21.7 




19.6 


21.4 


14.4 


F r (tc<) 


0.806 


0.796 


0.816 


0.806 




0.738 


0.754 


0.683 


f r\ 

(W/m -°C) 


4.458 


4.902 


4.006 


4.847 




4.470 


4.450 


4.445 


















o x 100 


0.564 


0.583 


0.552 


0.749 




0.539 


0.597 


0.6032 


n (%) 



















a is the mean residual error of the linear correlating curve 

n 

2 
Base case stagnation At/G rise = 0.17°C-m /W 

Samples of the inner cover , FEP film, showed apparent changes from 0.96 to 0.81 and 0.79 after 480 
days exposure. The transmittance changes in the inner cover resulted from an outgassing deposit on 
the inner surface which could be removed by washing. The results for the four degraded 
transmittance combinations are shown in the table. 

Comparing calculated with the measured changes in the efficiency curve parameters (section 4.2), the 
observed variations in many cases for F (xa) are of approximately the same order. In many other 
cases, particularly for F U , the measured changes are not consistent with those expected solely 
from changes in collector material properties. Data scatter in the collector efficiency measure- 
ments most likely obscured the observation of these changes. 

4.5 ABSORBER STAGNATION TEMPERATURE MEASUREMENT RESULTS 

The stagnation, or no-flow, absorber plate temperature was routinely monitored with embedded 
thermocouples located in the center of the plate for the collectors used in the series 1 tests. 
Separate research was also carried out at VPI&SU to investigate the potential of alternative test 
procedures to detect thermal performance changes based on such temperature measurements. The data 
obtained were used to investigate the actual effects on collector components and to compare relative 
advantages of test methods based on energy output or stagnation temperature measurement. 

4.5.1 General Considerations 

Temperature (both ambient and collector component), solar radiation (flux and spectral distribution), 
and moisture availability are considered to be the significant environmental factors contributing to 
corrosion or degradation in optical properties of the key components of solar collectors. Therefore, 
the selection of exposure conditions (laboratory or field) , the development of test procedures (real- 
time and accelerated) , and the evaluation of material properties after exposure to test conditions 
must include characterization of these factors. Long-term average and weather tape climatic data for 
specific sites in the United States are available from the NOAA [44]. Local differences in conditions 
between the test site and the weather bureau instrumentation, variations in real weather from year- 
to-year and the need for supplemental information to correlate with the particular collector com- 
ponents resulted in the need to perform measurement of real-time conditions at each site, including 
irradiance simulators and laboratory tests. 



108 



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hi 


CM 

hJ e 


X fr* 




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o 


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v^' 




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pi 3 


cr 




w 










b-> 


P* N_/ 


D 



E 

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cfl 

a 

to 



cfl 

CO 



109 



4.5.2 Temperature 

The absorber plate is considered to be one of the components of the collector most susceptible to 
damage because of the magnitude and range of temperatures experienced during operational and 
nonoperational periods. The temperature of the absorber plates during a typical nonoperational 
exposure day at site 2 and site 4 are shown in figures 4.5.1 and 4.5.2, respectively. The 
influence of absorber material and other collector construction details on the peak temperature is 
illustrated in figure 4.5.1 by the range from about 120°C for collector B to about 210°C for 
collector D. Similar temperature profiles for collectors B and D at site 4 obtained under summer 
and winter conditions indicate the large variations with ambient temperature and wind conditions 
for a non-selective, double-glazed collector with relatively large heat loss coefficients as 
compared to the essentially identical plate temperature exhibited by a double-glazed, selective- 
coated absorber with a low loss coefficient. 

A comparison of the absorber plate temperatures for collectors D when exposed at sites 1, 2, and 4 
and to the xenon and tungsten simulators is shown in figure 4.5.3. The higher peak temperature 
obtained in the tungsten simulator is attributed to the deviation in spectral distribution from the 
sun (larger infrared portion) and the higher environmental temperature. The higher peak 
temperature obtained in the xenon simulator is partially the result of the higher peak flux of 1100 
W/m as compared to about 1000 W/m experienced outdoors. The sharper rise and fall of the 
temperature in the simulator results from the abrupt turn-on and shut-off of the simulator lamps. 

Absorber plate and cover material temperatures and degradation are dependent upon the solar 
radiation flux and spectral distribution. Measurements of the spectral distribution were beyond 
the scope of this program except for the radiometric measurement of solar radiation below 
383 nm (ultra-violet) performed at site 1. Nonoperational exposure tests [33, 45-48] typically 
require exposures of at least 30 days at an irradiation level of 17,000 kJ/m2 (1500 Btu/ft 2 ) per day 
or greater. Additionally, one period of 4 hours or more with an irradiance greater than 947 W/m 
(300 Btu/hr-ft ) is generally required. Continuous solar irradiance measurements were made at each 
site and the total number of accumulated days in which the total daily radiation exceeded various 
levels was calculated for a one-year period as shown in figure 4.5.4. This figure can be used to 
estimate the probability and length of exposure time required to meet exposure tests with a wide 
range of locations in the contiguous United States. The figure shows that site 4 (Gaithersburg, 
MD) has a considerably lower potential for such tests as compared to the other three locations. 
Complete data for the measured daily solar irradiance are included in the appendix for all four test 
sites throughout the duration of the reliability/durability test program. 

4.5.3 Combined Irradiance and Ambient Temperature 

The current nonoperational test procedure requires a number of hours of simultaneous exposure to 
solar irradiance and ambient temperature [7], In order to evaluate the capability of the various 
sites to provide these minimum conditions and to obtain a data base for recommending 
modifications to current practice, measurements of irradiance and ambient temperature were analyzed 
over a one-year period for each site. The number of hours in which the exposure condition were 
less than the values of irradiance for 950, 1000, 1050, and 1100 W/m 2 and 15, 20, 25, and 30°C are 
shown for each site in tables 4.5.1 through 4.5.4. These data show that site 4 meets the ASTM E 823 
exposure condition of an irradiance of 950 W/m with an ambient temperature of 25°C only 9 hours 
per year. 

4.5.4 Use of Stagnation Temperature for Monitoring Changes in Thermal Properties 

An analytical and experimental investigation was carried out to evaluate an alternative to the 
energy output method for measuring thermal degradation of materials used in flat-plate collectors. 
The method, originally proposed by Birnbreier [49], is based on measuring the temperature of the 
absorber under a stagnation condition before and after prolonged' exposure. For a given solar 
irradiance level, the measured absorber stagnation temperature depends on cover transmittance, solar 
absorptance and infrared emittance of the absorber, and the collector loss coefficient. The method, 
test procedures, and results are discussed in detail in References [30-32, 50], References [30-31] 
are concerned with applying the stagnation temperature methods under nearly steady-state conditions 
while Reference [32] considers an averaging method to account for transients in the solar irradiance 
profile. 



110 



O COLLECTOR D, 

A COLLECTOR B, 

A COLLECTOR H. 

• COLLECTOR E. 




10 II 



12 I 2 

DAY TIME 



Figure 4.5.1. Typical absorber stagnation temperature profiles of exposed collectors, site 2, clear d 



ay 





250 


l 1 


i 


' 


, . , . T . __ ...^ — r - 














A 9/9/79 


O 2/3/80 






peok I »22"C 




peok l --3*C 




225 


- 






peak I, * 1041 W/m* 


peok I, »I028 W/m 2 


- 


o 





























— • 










— ^^^T^-tt^^^ COLLECTOR 0, 




LJ 


200 


- 




^^*» 


"""•oC^C/ 


- 


CC 








J^ 


\ TSw 




3 








,x 


</ ^**w. 




< 

or: 

LJ 


175 






Jr 


V\ 


- 


Q. 










\ \ 




2 










_^ \ \ 




UJ 

1- 


150 








^^ \ \ 




X. 




Ji 






^^V ^ \ 




UJ 










/\. N \ 




m 

CC 




/i 
// 






^-COLLECTOR B,-^ >. \ \ 




O 


125 


/' / 






/ ^V \ \ 


— 


CO 




/> / 






/ XN \ 




CD 
< 




/I / 
/> i/ 

fi r 

/ 1 / 




i 


—° «^ \\ \ 






100 


/ 1 / 
1 1 / 
/ 1 / 


^ 




V ^ \ 


\ - 






y 




\ ^ 






75 


7/ °" 

6/ 


1 


■ 


X 

i i i i i . 


- 



8 9 10 II 12 13 14 15 16 17 18 

TIME 

Figure 4.5.2. Daily absorber stagnation temperature profiles for double-glazed collectors B and D, 
site 4, clear day 



111 



UJ 

q: 

h- 
< 
tK 

i'u'Ji 

Q. 

s 

hi 



m 

o 
to 

CD 
< 





- 


l 1 


i 




T~ 




t ■■ 


- 


250 


~^- m " • ■ ■ » 


225 






/ / 












200 
175 


- 




/ v 








- 








'ii * 

// 

ij/ 


o—o XENON SIMULATOR 




W 






150 
125 




'/I 

//■/ 

J \ 


• • TUNGSTEN SIMULATOR 

COLLECTOR D, 
A— -A SITE #2 5/12/80 

(t » 26°C,I,(peak) = 1034 W/m') 
A A SITE # 1 1/31/80 

(l = 23 <> C,l,(peak) =1041 W/m*) 

D — D SITE # 1 9/20/79 
(» = 37 o C,I,(p«0k) =1021 W/m*) 




3 \ V 
\\ k 


\\ 

A 
\\ \ 

\ * \ 

V 


: 


100 
75, 


- / 

// 

// 


i 


■— ■ SITE * 4 2/3/80 

(t = -3 C,I,(peok)= 1028 W/m 2 ) 

i i i 




■ 


\ 



10 



12 



TIME 



Figure 4.5.3. Comparison of absorber stagnation temperature profiles using solar irradiance simulators 
and outdoor exposure, collector D 



ACCUMULATED DAYS ABOVE MINIMUM DAILY SOLAR 
RADIATION LEVELS FOR EACH SITE. 



PHOENIX, ARIZONA 




>4700 >5000 >5500 >6000 >6500 >7000 >7500 >8000 >8500 >9000 
TOTAL DAILY RADIATION (Wh/m*) 

Figure A. 5. 4. Days per year above minimum solar irradiance levels for each of the 4 test sites 



112 



Table 4.5.1 Hours of Exposure with Conditions Exceed- 
ing Corresponding Ambient Temperature and 
Irradiance Levels. Site 1, Phoenix, 
Arizona, July 1979 to June 1980. 





2 
Irradiance (W/m ) 


Ambient 
Temperature (°C) 


950 


1000 


1050 


1100 


15 


360.5 


113.25 


20.75 


2.75 


20 


335 


106.25 


17.5 


1.75 


25 


228.75 


70.5 


10.5 


1.75 


30 


134.25 


38.5 


7.75 


1.5 



Table 4.5.2 Hours of Exposure with Conditions Exceeding 
Corresponding Ambient Temperature and 
Irradiance Levels, Site 2, Cape 
Canaveral, Florida, November 1979 to 
October 1980. 





2 
Irradiance (W/m ) 


Ambient 
Temperature (°C) 


950 


1000 


1050 


1100 


15 


389 


177 


42.5 


14.0 


20 


324.5 


136 


27.5 


12.0 


25 


230.5 


77 


12.0 


7.5 


30 

. — . — . — — _ 


94.5 


19.5 


3.0 






113 



Table 4.5.3 Hours of Exposure with Conditions Exceeding 
Corresponding Ambient Temperature and 
Irradiance Levels, Site 3, Palo Alto, Cali- 
fornia, July 1979 to June 1980. 





2 
Irradiance (W/m ) 


Ambient 
Temperature (°C) 


950 


1000 


1050 


1100 


15 


453 


128 


4.5 





20 


291 


92 


3.0 





25 


173 


60 


2.0 





30 


38 


11.5 









Table 4.5.4 Hours of Exposure with Conditions Exceeding 
Corresponding Ambient Temperature and 
Irradiance Levels, Site 4, Gaithersburg, 
Maryland, April 1979 to March 1980. 





2 
Irradiance (W/m ) 


Ambient 
Temperature (°C) 


950 


1000 


1050 


1100 


15 


56.25 


22 


1.5 


0.25 


20 


35.5 


13.5 


1.0 


0.25 


25 


9.0 


4.0 


— 


— 


30 


2.5 


1.0 


— 


— 



114 



The advantages and limitations of the proposed method are briefly summarized here; the reader is 
referred to the references for detailed information. The investigations showed that the proposer! 
method is as sensitive to small changes in collector material properties as the currently used 
method based on measuring the energy output. Figure 4.5.5 shows the sensitivity of the new method to 
a range of property changes for typical collectors. In this figure, the abscissa is the stagnation 
temperature rise of the absorber above ambient normalized by the initial value. Property changes on 
the order of 0.1 in plate absorptance, cover solar transmittance, and plate emittance would be 
detectable using the new method with the exception of cover transmittance for collector E which would 
be only marginally detectable. While the measurements required in the stagnation temperature method 
are much simpler than those required to measure energy output, other factors, such as nonisothermal 
absorbers, variations in environmental conditions, and transient response, must be taken into 
account. Figure 4.5.5 assumes that the environmental conditions are essentially constant and that 
steady-state conditions exist. Figure 4.5.6 shows the variation of normalized stagnation temperature 
rise as a function of environmental parameters for collectors D and E. Clearly, wind speed, ambient 
temperature, and solar irradiance have the largest effect on measured stagnation temperature. These 
parameters, however, primarily affect U rather than (tcx) . 

The investigation showed that even small transients in the solar irradiance profile greatly compli- 
cates the determination of small property changes. Reference [32] describes a method for reconciling 
problems arising from short-term transients in the solar irradiance profile as well as long-term 
variations in daily solar radiation. This method is based on measuring the absorber temperature 
continuously over a period of several days along with the total daily solar irradiation. The 
absorber temperature rise above ambient is then integrated to determine a daily value. The measure- 
ments provide data for a graph of the integrated absorber temperature parameter vs total daily 
irradiation in the collector aperture. Comparison of two such graphs, based respectively on data 
obtained over a period of several days before and after a prolonged durability exposure, show if the 
thermal properties of the collector have changed significantly. Using this approach,' 
calculations showed that the effect of short-term transients in the daily irradiance profile is 
insignificant. Figure 4.5.7 shows expected results from using all-day integrated parameters for 
stagnation temperature and solar irradiation for a typical change in plate absorptivity. 

The investigation showed that the all-day integration method is a viable approach for detecting 
changes in material properties that has advantages over alternative test methods based on steady- 
state measurements of either absorber stagnation temperature or collector energy output. 
Previously, the temperature measurement approach was limited to short time periods near solar noon 
on clear days outdoors or to using solar irradiance simulators indoors. Outdoor test data over a 
relatively short period of time was obtained to validate the calculated results and depict typical 
expected results using the test procedure. The principle limitation of both stagnation temperature 
methods, however, is the strong effect of other environmental conditions, particular wind speed, on 
the test results. It should also be noted that neither the temperature nor energy output 
measurement method can directly identify the particular material property that changed in a 
collector. With limitations on wind speed and for relatively clear days, the preliminary 
investigation suggests that stagnation temperature rise is a reproducible collector performance 
parameter which is at least as sensitive as the ASHRAE 93-77 method and much less expensive to 
measure. The method is applicable to a broad range of collector designs and can be extended to form 
the basis for simple comparative materials tests. 

4.6 GENERAL OBSERVATIONS ON OUTDOOR TEST METHODS FOR MEASUREMENT OF COLLECTOR PERFORMANCE 
DEGRADATION 

Except for catastrophic failures and some obvious problems with a few plastic glazing materials, the 
test program collectors, which are typical of modern commercial equipment, were quite durable in 
thermal performance and held up very well under unusually adverse nonoperational conditions over a 
period of approximately 3 years. 

Neither the energy output measurement method or alternative methods based on measuring stagnation 
temperature are entirely satisfactory for determining the typically small property changes in 
materials that occurred during the test period. The stagnation temperature method involved simpler 
measurements and would be less expensive. However, this method has neither been validated extensively 
nor subjected to broad field experience. While the energy output measurement is widely used and 
accepted by the solar energy community for assessing solar collector thermal performance, variations 
in measured performance parameters are higher than is desirable for direct comparison of results to 
determine degradation in collector material properties. 



115 



1.0 



0.9 



x 



X 



0.8 - 



0.7 



- 


1 1 


1 


- 




-*C^\ Af c , COLLECTOR E 


— 




Ae p , COLLECTOR D - 


- 


COLLECTOR D N, 
f c = 0.94. a p »0.94. € p = 0.25 


S. n\ Aa p , COLLECTOR D 


- 


COLLECTOR E 


>v Aa p . COLLECTOR E 


- 


f c = 0.86, 5 p *0.95 


N. 


- 




\ AT C , COLLECTOR D 


- 


1 I 


1 



0.0 



0.1 0.2 0.3 

PROPERTY CHANGE (-Aa p ,-AT C , A€ p ) 



0.4 



Figure 4.5.5. Sensitivity of normalized absorber stagnation temperature to changes in material properties 



116 



CO 

X 




0.9 - 



IVV 



CO 

X 

\ 

X 



0.9 - 



I 



J L 



15 20 

y/«,)g(DEG)| 



20 



30 



40 50 

SLOPE(DEG). 




25 



60 



f 



5 

i 


10 

1 


15 

i 


20 25 
, T (C) , 


30 

1 


35 

i 


40 

i 




i 


1 


2 


3 A 
,V>A) , 


5 

i 


6 
i 


7 
I 


-25 

i 


-20 

i 


-15 


-10 -5 
,(T t -T )(C), 



i 


5 

1 


10 

I 



30 



35 



70 



800 



650 



900 



950 



1100 



1150 



1000 1050 

6(W/m 2 ) 

Figure 4.5.6. Effect of environmental test conditions on normalized absorber 
stagnation measurements 



117 



o 



H° 



ib" 







1 1 1 


1 1 1 






COLLECTOR B 






THEORETICAL VALUES 






1000 




$«40°, T =20C, V w »3.0m/8 
O INITIAL PERFORMANCE 


r— a p =096 sy 








D DEGRADED PERFORMANCE 




_ 








j^^^ L-a p =0.86 


- 










— 


500 








- 







1 1 1 


i i i 





10 



/ 



24 h 

Gdt 



20 



(MJ/m ) 



30 



Figure 4.5.7. Sensitivity of the all-day integration method to a 0.10 change 
in plate absorptance for collector B 



118 



The precision of the measured thermal performance parameters, F (ta) and FrU^, obtained in this 
reliability/durability test program are consistent with the experimental error inherent in the 
current ASHRAE 93-77 test method [6], For comparison, the variation in F ( to) and F^Ut is somewhat 
better than in the original roundrobin test program [39] conducted by NBS. In that program, the 
coefficient of variation (standard deviation in percent of mean value) was 4.6 - 7.7 percent 
for F r (tcx) and 16 - 25 percent for FrU^. In a subsequent Department of Energy comparability test 
program [51], coefficients of variation for F (tot) ranged from 2.0 - 6.0 percent and ranged from 3.0 
- 18 percent for F^U^. In the current test program, the coefficient of variation for the initial 
testing was 1.5 - 4.0 percent for F R (ta) and 5-13 percent for F R U L . The data from these three 
test programs and the supporting analysis of the various factors which could affect the data show 
that the variations in the thermal performance results presented herein are within the precision 
limits of the current ASHRAE 93-77 test standard. 



119 



5. CONCLUSIONS AND RECOMMENDATIONS 

In this section, the significant observations and recommendations resulting from this test program are 
summarized. These findings are primarily concerned with an evaluation of test methods for determining 
the thermal performance and durability of solar collectors and their materials. During the course of 
this investigation, significant research findings have been published in the open literature and pre- 
sented to appropriate organizations concerned with the development of standards for solar collectors 
and their materials such as ASTM and ASHRAE. 

5.1 COVER MATERIALS 

• Outdoor exposure at sites having a combination of high prevailing humidity and high solar 
radiation generally produced more severe changes in polymeric cover materials than the 
other test sites. The most extensive microcracking and optical transmission losses 
occurred during the outdoor exposure of full-size solar collectors and cover mini-boxes at 
the test sites located in Cape Canaveral and Palo Alto. Next in order of severity were 
changes observed at the Gaithersburg site followed by those at the Phoenix site. This is 
consistent with the findings of Clark and Roberts [52] for cover specimens exposed in 
Florida and Arizona. It is most likely due to the combined effects of photodegradation 

and cyclic shrinking and swelling of the polymer surface due to moisture wetting and drying. 
It Is essential that outdoor exposure testing include the combinations of environmental 
exposure parameters that will occur in normal use. 

2 

• Outdoor "real time" exposure on cover mini-boxes for 480 days (^ 17,000 kJ/m /day) 

is required for many polymeric materials in order to induce degradation that is 
detectable without sophisticated analysis. This is equivalent to two or more calendar 
years. Changes such as microcracking and embrittlement were not readily observable in 
shorter periods of time for many materials. 

• Accelerated outdoor exposure of polymeric cover materials for 120 calendar days produced 
changes similar to those occurring in 480 days of "real time" exposure (>/ 17,000 kJ/m^/day ) . 
The cover specimens were mounted on the accelerated exposure cover mini-boxes shown in 
figure 2.2.2 of this report and exposed to concentrated sunlight (~ 6 suns) and an inter- 
mittent water spray using the apparatus described in ASTM E 838 [36]. 

• Outdoor exposure of cover materials at elevated temperatures representative of both 
operational and stagnation conditions is needed to assess the durability of polymeric 
covers of solar collectors . Elevated temperatures have an accelerating influence on the 
degradation of polymeric materials. In addition, exposure to stagnation temperatures 
resulted in the loss of ultraviolet radiation screening additives from several of the 
materials studied. Another consideration is that many polymers are more susceptible to 
degradation when heated above their glass transition temperature. The cover temperatures 
in stagnating solar collectors are higher than the glass transition temperatures of many 
polymers. The "real time" cover mini-boxes used in this test program produced cover 
temperatures representative of operational conditions in full-size flat-plate solar 
collectors. By controlling the cooling air flow in the accelerated outdoor test apparatus, 
it was possible to attain cover temperatures representative of stagnation conditions. 



• 



Currently recognized indoor laboratory exposure tests are not capable of reproducing 
many of the changes observed in this program for polymeric materials exposed outdoors . 
The indoor laboratory exposure testing conducted in this test program, which used test 
procedures similar to those in ASTM E 765 [8], did not duplicate the extensive micro- 
cracking observed outdoors. Other testing for 1200 hours using a xenon arc weathering 
machine and an intermittent water spray, as specified in ASTM E 765, was also unsuccessful 
in duplicating this microcracking. In addition to causing light transmission losses, 
microcracking is of particular importance when thin film polymers are used as glazing 
because of its influence on mechanical properties. Yamasaki and Blaga [53] have had 
some success in using an indoor test to duplicate the surface degradation occurring outdoors 
on glass-fiber reinforced polyester sheets. However, several thousand hours of exposure 
in a weathering machine is required. 



120 



• Heat stability testing of cover materials at a temperature of 90° C in addition to the 
temperatures currently specified in ASTM E 765 [8] is desirable. ASTM E 765 
currently specifies the use of two temperatures for screening purposes; 75° C for outer 

and single cover materials and 125° C for inner cover materials. Mathematical modelling has 
shown that the covers of single-glazed flat-plate solar collectors can reach 90°C under 
stagnation conditions with zero wind speed and solar radiation levels of 1000 W/nr . Since 
the degradation of polymeric materials is accelerated by exposure to elevated temperatures, 
it is desirable to test covers for single-glazed collectors at 90°C. 

• Accelerated aging of cover materials using xenon arc weathering machines should be 
performed at temperatures representative of stagnation conditions . Materials exposed to 
xenon arc radiation at 90° C, the temperature of single glazing in stagnating flat-plate 
solar collectors, had considerably more degradation than samples of the same materials 
exposed at 70° C. Another consideration is the loss or degradation of ultraviolet radiation 
screening additives at elevated temperatures. 

• Exposure testing of polymeric glazings using elevated temperature and humidity conditions 
produced changes considerably different from those caused by outdoor exposure of samples 
of the same materials. The primary value of this type of long-term test would be for 
glazings of trickle down collectors and polymeric storage tanks where continuous exposure 
to moisture at elevated temperatures is likely. As was previously mentioned, there is 

a need for the development of an aging test which takes into account the synergistic 
effects of moisture, temperature, and sunlight. Cyclic moisture exposure to cause shrink- 
ing and swelling of the polymer should be considered as part of such a test. 

• Normal and hemispherical spectral transmittance curves measured in the ultraviolet - 
visible spectral region are more sensitive indicators of cover materials degradation than 
integrated spectral transmittance values determined in accordance with ASTM E 424 [34]. 
Emphasis in current ASTM methods concerned with the durability of cover materials [8, 9, 
11] has been placed on the use of integrated spectral transmittance values for measuring 
optical property changes in glazings. These integrated values are indicators of changes 
in the engineering properties of glazing materials and provide the type of data required 
by solar collector designers. However, they are not sensitive to spectral changes that 
occur in a limited part of the solar spectrum, i.e., at short wavelengths in many 
polymers. Since little or no energy is found in the solar spectrum in this short 
wavelength region, integrated spectral transmittance measurements are of little value in 
detecting these changes which are sensitive indicators of degradation in many polymers. 
More emphasis should be placed on the analysis of spectral curves. 

• Normal ultraviolet-visible spectral transmittance measurements have greater sensitivity 
to chemical and/or physical changes in cover materials than hemispherical transmittance 
spectra . The normal measurements are a sensitive indicator of changes in light scattering. 

• There is a need for data on the infrared spectral properties of organic materials; e.g. 
emittance and diffuse transmittance . These data are needed for the mathematical modelling 
of collector designs and the calculation of exposure temperatures. 

5.2 ABSORBER MATERIALS 

• Outdoor exposure of small-scale absorber samples mounted on the absorber of a simulated 
collector is an effective method for determining the thermal stability of large numbers 
of samples under stagnation conditions; however, it is not a valid test for determining 
the stability in the presence of moisture of these materials when used in solar 
collectors . In both ASTM E 781 [13] and this test program, care was taken to prevent 
moisture penetration at joints, seams, and seals. However as was observed in this test 
program, moisture penetration in full-size solar collectors appears to be a common 
occurrence. This moisture penetration resulted in corrosion and appearance changes not 
observed with small-scale specimens. 

• Both the simulated solar collectors discussed above and the accelerated exposure mini- 
boxes used in this test program (see figure 2.2.5) are useful for exposing small 
absorber test specimens to stagnation temperatures; however, unlike the case with 



121 



cover materials, it is not clear that exposure to concentrated solar radiation accelerated 
the photolytic degradation of absorber materials. Similar changes were observed for test 
specimens exposed to concentrated sunlight in the mini-boxes and to "real time" conditions 
in the simulated collectors. 

• Indoor laboratory exposure of absorber test specimens to temperatures characteristic of 
stagnating solar collectors is an effective method for determining their thermal stability. 
Temperature exposure produced thermal changes similar to those which occurred in the 
outdoor exposure of absorber test specimens. 

• Exposure testing of absorber materials with continuous elevated temperature and humidity 
exposure conditions, such as those used in this test program, produced changes that are 
considerably more severe than those produced by the outdoor exposure of materials of the 
same composition in full-size solar collectors and exposure test boxes. The moisture 
exposure conditions specified in ASTM E 744 [12], 90°C and 95 percent relative 
humidity for 30 days, need to be reexamined since these conditions, which are similar 

to those used in this test program, may be unduly severe. Materials which are 
severely degraded as a result of exposure using these test conditions may still be 
capable of providing adequate service as absorbers in flat-plate solar collectors. 

• Exposure to xenon arc radiation at elevated temperature produced significant optical 
property changes in several of the selective absorber materials studied . Exposure 
testing with xenon arc radiation should be performed at temperatures representative 
of solar collector operating conditions since degradation rates are generally 
accelerated by elevating the exposure temperature. The spectral transmission 
characteristics of the covert s) used in a solar collector will control the amount, 
and spectral distribution, of the solar radiation reaching the absorber surface. For 
this reason, testing should be performed with the glazings to be used in the actual col- 
lector installed between the light source and the absorber surface, or with the worst-case 
configuration possible. The light source should be filtered to match the solar 
spectrum. 

• The thermal cycling test most closely simulated the types of corrosion and other 
changes that were observed in full-size solar collectors . In this test, coupon 
specimens were removed from a chamber at -10°C and allowed to equilibrate at room 
temperature prior to being placed in an oven at 177°C. During this equilibration 
process, moisture condensed on the test specimen surfaces; this wetting most likely led 
to the corrosion observed. Because of the apparent importance of the condensed 
moisture, the humidity needs to be controlled during the equilibration process. 

• Research needs to be conducted to define the extent to which the absorber materials 

in operational solar collectors are exposed to moisture . Factors such as wetting time, 
absorber temperature, diurnal breathing, and environmental exposure conditions need 
to be considered in this research. The problem of how to determine the proper test 
conditions for assessing the moisture stability of absorber materials is very complex, 
i.e., the presence of moisture on the inner surface of the glazing of a collector that is 
stagnating on a clear day does not mean that the relative humidity in the vicinity of the 
absorber is anywhere near as high as that in the vicinity of the glazing, since absorber 
temperatures are much higher than cover temperatures. In addition, the presence of 
porosity in many absorber coatings means that moisture can condense in these pores at 
humidities lower than 100 percent relative humidity. It is more likely that moisture would 
condense out on the absorber at night when it is cool rather than in the daytime. The 
problem is further exacerbated by the presence of moisture due to water leakage in many 
solar collectors, in addition to condensation caused by diurnal collector breathing. 

• Spectral reflectance curves are a more sensitive indicator of absorber materials 
degradation than solar absorptance values determined by integrating over a standard 
solar energy distribution curve . Current ASTM standards concerned with the durability 
of absorber materials [12, 13] rely on the use of integrated values determined in 
accordance with ASTM E 424 [34], Spectral reflectance changes in the near infrared 
region are a sensitive indicator of early degradation in many absorber materials. Since 
the solar spectrum contains very little energy in this region, integration tends to 
conceal these changes. 



122 



• Absorber materials generally exhibited larger changes in emittance than in integrated 
solar absorptance for both the outdoor exposure and indoor laboratory tests . Of the 
thirteen different types of absorber materials studied, the aluminum oxide conversion 
coating, material N, was the only exception to this trend. 

• The absorber samples tested must duplicate the full-scale collector material in 
substrate, preparation, and coating application techniques to provide valid test results . 
The variability of black chrome performance with processing parameters is an example of 
this type of consideration. 

5.3 SOLAR COLLECTORS 

• Neither energy output measurements, based on ASHRAE Standard 93-77, nor stagnation 
temperature measurement methods are satisfactory for determining changes in material 
properties observed after outdoor exposure . Collector thermal performance measurements 
are not sufficiently sensitive and precise under typical test environments to detect the 
relatively small changes in efficiency curve slope and intercept resulting from 
changes in material properties (cover transmittance, absorber absorptance and emittance, 
and insulation thermal conductivity) caused by 480 days of outdoor exposure 

(> 17,000 kJ/m 2 /day). 

• The absorber stagnation temperature methods evaluated during this investigation are 
at least as good an indicator of solar collector thermal performance changes, with 
the exception of bond conductance, as the ASHRAE 93-77 test method and are less 
expensive to implement. 

• Initial reliability/durability screening tests for materials considered for use in 
solar collectors should be performed both in the laboratory and outdoors on small- 
scale samples using materials exposure tests such as those used in this program . 
These small-scale tests can be used to narrow down the choice of materials. Careful 
field measurement of peak stagnation temperatures for full-size collectors or 
extrapolation of thermal performance curves should be used as a basis for determining 
appropriate exposure test conditions. 

• The final selection of materials for use in solar collectors should be based both on 
the results of small-scale materials tests and on an evaluation of the properties of 
materials samples taken from full-size solar collectors exposed outdoors . Materials 
should be capable of withstanding stagnation conditions without property changes greater 
than those allowed for in the collector design. Exposure testing of full-size collectors 
makes possible an evaluation of materials interactions that may not be obvious from small- 
scale materials tests. Additional consideration needs to be given to changes in the mechan- 
ical properties of solar collector materials; especially thin film glazings. 

The uncertainty in the measured efficiency curve intercept and slope parameters are as 
expected considering the inherent precision in the ASHRAE 93-77 test method . The 
coefficients of variation for the initial baseline tests of the eight collector types, 
from all test sites, were from 1.5 to 4.0 percent for the intercept parameter and 5 to 13 
percent for the slope parameter. Statistical analyses of the test data showed no systematic 
trends, either higher or lower, with the site performing the testing or with individual 
collector test stands at a particular test site. 

Intercept values determined from linear first-order correlations of efficiency data 
provided good agreement with those obtained from second-order correlations . The effect 
on the intercept was less than one percent in all cases, with the linear curve always 
giving an intercept higher than the second-order curve. 

For flat-plate collectors of conventional design, calculation procedures are capable 
of giving results at least as good as those obtained with the ASHRAE 93-77 measure- 
ment procedure for the incident angle modifier . A coefficient of variation for the 
incident angle modifier parameter of 34 percent was found for measurements made on 
solar collectors of the same type at three test sites. 

• A comparison of results obtained under natural outdoor test conditions with those 
obtained using tungsten-halogen and xenon arc simulators showed that higher 



123 



• 



• 



thermal performance results can be obtained with the solar irradiance simulators for 
collectors having polymeric covers . The highest results were obtained with the xenon arc 
simulator. 

• Stagnation exposure testing of flat-plate solar collectors intended for use in systems 
with solar radiation augmentation reflectors should be conducted with these reflectors 
in place . Stagnation exposure testing of flat-plate solar collectors, in this 
program, for 60 days with reflectors (^ 17,000 kj/m^/day measured in the plane of the 
collector without reflectors) appeared to cause more severe thermal degradation than 
480 days (^ 17,000 kj/m 2 /day) of stagnation exposure of the same types of collectors 
without the reflectors. 

® The peak stagnation temperature for flat-plate solar collectors should be measured at a 
distance approximately one-fourth of the way below the top of the collector and with the 
collector tilted so that its aperture is normal to the sun . The magnitude of the peak 
temperature needs to be taken into account for both safety and durability considerations 
when selecting materials to withstand stagnation conditions. Measurements made as part of 
this test program [50] have shown that absorber and cover stagnation temperatures vary 
as a function of tilt angle and position on the absorber. This is most likely due to 
the combination of thermal stratification of air inside the collector and edge losses. 

• Over a period of approximately three years, pyranometer sensitivity changes, if not 
calibrated out, could give rise to errors comparable to the uncertainty in the measured 
collector efficiency . These pyranometer calibration changes are not linear with respect 
to exposure time. Temperature dependence of the pyranometers, which had built-in 
temperature compensation circuits, was not found to be significant. 

• There is a need for a water leakage test for flat-plate solar collectors similar to the 
test described in ASTM E 331 [54] for exterior windows, curtain walls, and doors . This 
method includes the use of a pressure differential, similar to that occurring with wind 
driven rain, to enhance the penetration of water into the assembly being tested. This 
test method could be simplified for flat-plate collectors by applying a negative pres- 
sure inside the collector case to induce the pressure differential. 

Several of the collectors used in this test program and others observed in the field showed 
signs of excessive internal moisture which appeared to result from leakage rather than 
condensation caused by diurnal collector breathing. The presence of moisture and 
resulting degradation of materials appears to be strongly dependent on both the design 
of collector joints and seams and the quality of workmanship in assembling the 
collectors. In several cases where two collectors of the same type were exposed side 
by side, only one of the two absorbers showed significant corrosion. Materials used in 
flat-plate collectors should be moisture resistant unless the collector is hermetically 
sealed. 

• Outgassing deposits were observed on the glazings of virtually all of the solar collectors 
exposed in the test program . However, changes due to this outgassing could not be 
discerned by thermal performance measurements. The outgassing was especially severe 

in the case of collector H. All of the paint absorber coatings studied in this program 
caused a buildup of outgassing deposits on the glazing of absorber test box in which 
they were exposed. Other potential sources of outgassing include insulation materials, 
gaskets and sealants, and adhesives. 



9 



The thermal shock/water spray test did not cause thermal shock problems in any of the 
solar collectors subjected to this test in this program. 



124 



6 . REFERENCES 

1. "Solar Heating and Cooling Demonstration Act of 1974," Public Law 93-409, September 3, 1974. 

2. "Solar Energy Research, Development and Demonstration Act," Public Law 93-473, October 26, 1974. 

3. "National Program for Solar Heating and Cooling of Buildings," Report No. ERDA 76-6, Energy 
Research and Development Administration (now Department of Energy), Washington, DC 20545, 
November 1976. 

4. "Methods of Testing to Determine the Thermal Performance of Solar Domestic Water Heating 
Systems," ASHRAE Standard 95-1981, American Society for Heating, Refrigerating, and Air- 
Conditioning Engineers, Inc., 1791 Tullie Circle, N.E., Atlanta, GA, December 1981. 

5. "Methods of Testing Thermal Storage Devices Based on Thermal Performance," ASHRAE 
Standard 94-77, ibid., February 1977. 

6. "Methods of Testing to Determine the Thermal Performance of Solar Collectors," ASHRAE 
Standard 93-77, ibid., February 1977. 

7. "Practice for Nonoperational Exposure and Inspection of a Solar Collector," ASTM E 823-81, 
American Society for Testing and Materials, 1916 Race St., Philadelphia, PA 19103. 

8. "Practice for Evaluation of Cover Materials for Flat-Plate Solar Collectors," ASTM E 765-80, 
ibid. 

9. "Practice for Exposure of Cover Materials for Solar Collectors to Natural Weathering Under 
Conditions Simulating Operational Mode," ASTM E 782-81, ibid. 

10. "Practice for Determining Resistance of Solar Collector Covers to Hail by Impact with Propelled 
Ice Balls," ASTM E 822-81, ibid. 

11. "Practice for Exposure of Solar Collector Cover Materials to Natural Weathering Under Conditions 
Simulating Stagnation Mode," ASTM E 881-82, ibid. 

12. "Practice for Evaluating Solar Absorptive Materials for Thermal Applications," ASTM E 744-80, 
ibid. 

13. "Practice for Evaluating Absorptive Solar Receiver Materials When Exposed to Conditions 
Simulating Stagnation in Solar Collectors with Cover Plates," ASTM E 781-81, ibid. 

14. "Specification for Rubber Seals Used in Flat-Plate Solar Collectors," ASTM D 3667-78, ibid. 

15. "Specification for Rubber Seals Used in Concentrating Solar Collectors," ASTM D 3771-79, ibid. 

16. "Specification for Rubber Seals Contacting Liquids in Solar Energy Systems," ASTM D 3832-79, 
ibid. 

17. "Specification for Rubber Seals Used in Air-Heat Transport of Solar Energy Systems," 
ASTM D 3903-80, ibid. 

18. "Specification for Rubber Hose Used in Solar Energy Systems," ASTM D 3952-80, ibid. 

19. "Practice for Laboratory Screening of Metallic Containment Materials for Use with Liquids in 
Solar Heating and Cooling Systems," ASTM E 712-80, ibid. 

20. "Practice for Simulated Service Testing for Corrosion of Metallic Containment Materials for Use 
with Heat Transfer Fluids in Solar Heating and Cooling Systems," ASTM E 745-80, ibid. 

21. "Practice for Screening Polymeric Containment Materials for the Effects of Heat and Heat 
Transfer Fluids in Solar Heating and Cooling Systems," ASTM E 862-82, ibid. 

22. Waksman, D. , Streed, E. R. , and J. Seiler, "NBS Solar Collector Durability/Reliability Test 
Program Plan," NBS Technical Note 1136, National Bureau of Standards, Washington, DC 20234, 
January 1981. 



125 



23. Streed E. R. and Waksman, D., "NBS Solar Collector Durability/Reliability Program," Durability 
of Building Materials and Components , ASTM STP 691, P. J. Sereda and G. G. Litvan, Eds., 
American Society for Testing and Materials, 1980, pp. 239-249. 

24. Streed E. R. , and Waksman, D., "Uncertainty in Determining Thermal Performance of Liquid-Heating 
Flat-Plate Solar Collectors," Journal of Solar Energy Engineering , ASME, Vol. 103, May 1981, 

pp. 126-134. 

25. Streed E. R. and Waksman, D., "Uncertainty in Determining Thermal Performance of Liquid-Heating 
Flat-Plate Solar Collectors," NBS Technical Note 1140, National Bureau of Standards, Washington, 
DC 20234, March 1981. 

26. Thomas, W. C. , Dawson, A. G., Ill, Waksman, D., and Streed, E. R. , "Incident Angle Modifiers 
for Flat-Plate Solar Collectors: Analysis of Measurement and Calculation Procedures," Journal 
of Solar Energy Engineering , ASME, Vol. 104, November 1982, pp. 349-357. 

27. Thomas, W. C, Dawson, A. G., Ill, Waksman, D., and Streed, E. R. , "Determination of Incident 
Angle Modifiers for Flat-Plate Solar Collectors," Proceedings of ASME Solar Energy Division: 
Fourth Annual Conference , April 1982, pp. 501-510. 

28. Streed, E. R. , Waksman, D., Dawson, A. G., Ill, and Lunde, A., "Comparison of Solar Simulator 
and Outdoor ASHRAE Standard 93 Thermal Performance Tests," Proceedings of 1980 Annual Meeting, 
American Section of the International Solar Energy Society , June 1980, pp. 405-409. 

29. Waksman, D., Streed, E. R. , and Dawson, A. G., Ill, "The Influence of Environmental Exposure on 
Solar Collectors and Their Materials," Proceedings of 1980 Annual Meeting, American Section of 
the International Solar Energy Society , June 1980, pp. 415-419. 

30. Dawson, A. G. , III, Thomas, W. C, and Waksman, D., "Evaluation of Absorber Stagnation 
Temperature as a Characteristic Performance Parameter of Flat-Plate Solar Collectors," ASME 
Paper 82-WA/Sol-5, American Society of Mechanical Engineers, New York, NY 10017, November 
1982. 

31. Dawson, A. G., Ill, Thomas, W. C, and Waksman, D.. "Solar Collector Durability Evaluation by 
Stagnation Temperature Measurements," Journal of Solar Energy Engineering , ASME, Vol. 105, 
August 1983, pp. 259-267. 

32. Thomas, W. C, Dawson, A. G., Ill, and Waksman, D., "Testing Solar Collector Materials 
Durability by Integrated Day-Long Stagnation Temperature Measurements," Proceedings of ASME 
Solar Energy Division: Fifth Annual Technical Conference , April 1983, pp. 301-307. 

33. Waksman, D., Streed, E. R., Reichard, T. W., and Cattaneo, L. E., "Provisional Flat-Plate Solar 
Collector Testing Procedures: First Revision," NBS Interagency Report 78-1305A, National Bureau 
of Standards, Washington, DC 20234, June 1978. 

34. "Test for Solar Energy Transmittance and Reflectance (Terrestrial) of Sheet Materials," 
ASTM E 424-71, op. cit. 7. 

35. "Test for Total Normal Emittance of Surfaces Using Inspection-Meter Techniques," ASTM E 408-71, 
op. cit. 7. 

36. "Practice for Performing Accelerated Outdoor Weathering Using Concentrated Natural Sunlight," 
ASTM E 838-81, op. cit. 7. 

37. Newland G. C. and Tamblyn, J. W. , " Mechanism of Ultraviolet Stabilization of Polymers by 
Aromatic Salicylates," Journal of Applied Polymer Science , _8, 1964, pp. 1949-1956. 

38. Moore, S. W. , "Los Alamos Optical Materials Reliability, Maintainability, and Exposure Testing 
Program," Report No. LA 9735 MS, Los Alamos National Laboratory, Los Alamos, New Mexico, 1983. 

39. Streed, E. R. , Thomas, W. C, Dawson, A. G., Ill, Wood, B. D., and Hill, J. E., "Results and 
Analysis of a Round-Robin Test Program for Liquid-Heating Flat-Plate Solar Collectors," NBS 
Technical Note 975, National Bureau of Standards, Washington, DC 20234, August 1978. 



126 



40. Thomas, W. C, "Solar Collector Test Procedures: Development of a Method to Refer Measured 
Efficiencies to Standardized Test Conditions," Report VPI-E-80.23, Virginia Polytechnic 
Institute and State University, Blacksburg, VA 24061, November 1980. 

41. Cheng, H. and Bannerot, R. B., "On the Weathering of Thin Plastic Films," ASME Paper 8I-WA/S0I-6, 
American Society of Mechanical Engineers, New York, NY 10017, November 1981. 

42. Douro, D. D., "Investigation of Performance Degradation and Test Methods for Flat-Plate Solar 
Collectors," M.S. Thesis, Mechanical Engineering Department, Virginia Polytechnic Institute and 
State University, Blacksburg, VA 24061, June 1982. 

43. Culkin, D. S., "Analysis of Flat-Plate Solar Collector Durability Test Data," M.S. Thesis, 
Mechanical Engineering Department, Virginia Polytechnic Institute and State University, 
Blacksburg, VA 24061, June 1982. 

44. "Hourly Solar Radiation Surface Meteorological Observations," TD-9724, S0LMET, National Oceanic 
and Atmospheric Administration (NOAA), Asheville, NC, 1979. 

45. "Test Methods and Minimum Standards for Certifying Solar Collectors," ISCC Document 80-1, 
Interstate Solar Coordination Council, December 1980. 

46. "Test Methods and Minimum Standards for Certifying Solar Collectors," SRCC Standard 100-81, 
Solar Rating and Certification Corporation, 1001 Connecticut Avenue, NW, Washington, DC, March 
1982. 

47. "Standard for Solar Collectors," ARI Standard 910-81, Air-Conditioning and Refrigeration 
Institute, 1815 North Fort Myer Drive, Arlington, VA 22209, 1981. 

48. "Intermediate Minimum Property Standards Supplement - Solar Keating and Domestic Hot Water 
Systems," 1977 Edition, prepared for HUD by NBS. Available from GPO, Order No. 

SN 023-000-90161-7. 

49. Birnbreier, H. , "Durability Tests by Stagnation Temperature Measurements," International 
Energy Agency Task III meeting, Heidelberg, West Germany, December 1978. 

50. Dawson, A. G. , III, "Stagnation Temperature Test Methods for Determining Solar Collector 
Thermal Performance," Ph.D Dissertation, Mechanical Engineering Department, Virginia Polytechnic 
Institute and State University, Blacksburg, VA 24061, June 1981. 

51. Kirkpatrick, D. L., "Flat-Plate Solar Collector Performance Data Base and User's Manual," 
Report No. SERI/STR-254-1515, Solar Energy Research Institute, Golden, CO 80401, July 1983. 

52. Clark, E. J. and Roberts W. E. , "Weathering Performance of Cover Materials for Flat Plate 
Solar Collectors," NBS Technical Note 1170, National Bureau of Standards, Washington, DC, 
20234, November 1982. 

53. Yamasaki, R. S. and Blaga,, A., "Accelerated Weathering Test for Glass Fiber Reinforced 
Polyester (GRP) Sheets," Durability of Building Materials and Components , ASTM STP 691, 
P. J. Sereda and G. G. Litvan, Eds., American Society for Testing and Materials, 1980, 
pp. 874-889. 

54. "Standard Test Method for Water Penetration of Exterior Windows, Curtain Walls, and Doors 
by Uniform Static Air Pressure Difference," ASTM E 331-70, American Society for Testing and 
Materials, 1916 Race Street, Philadelphia, PA 19103. 



127 



APPENDIX A: SOLAR RADIATION EXPOSURE SUMMATIONS 

Table A.l Exposure Summation for all Collectors 

Table A. 2 Exposure Summation for Material Samples at Phoenix 

Table A. 3 Exposure Summation for Material Samples at Cape Canaveral 

Table A. 4 Exposure Summation for Material Samples at Palo Alto 

Table A. 5 Exposure Summation for Material Samples at Gaithersburg 



129 



Table A. 1. 



Co I I ector 
ID 



Exposure Summation for all Collectors (MJ/sq m) 
3 15 30 



Exposure Days* 

60 120 



240 



480 



Tota I 
Days* 



Tota I 
Exposure 



A- 1-1 


82 


414 


775 


1527 


1 3293 


6594 


1 13118 


I 485 


1 13251 


A-1-2 


82 


414 


770 


1525 


3293 


6592 


13118 


I 485 


I 13250 


A-1-3 


86 


411 


774 


1678 


1 3244 


6524 





I 250 


1 6805 


A-l-4 


80 


399 


775 


1570 











I 63 


1 1639 


B-1-1 


81 


412 


772 


1575 


3305 


6626 


13151 


I 488 


I 13363 


B-1-2 


78 


407 


755 


1563 


3303 


6624 


13149 


485 


I 13282 


B-1-3 


86 


411 


774 


1678 


3244 


6524 





250 


I 6805 


B-1-4 


80 


399 


775 


15 70 











63 


1639 


C-l-1 


81 


412 


769 


1525 


3263 


6591 


13139 


487 


1 13324 


C-1-2 


78 


406 


756 


1511 


3277 


6614 


13129 


484 


1 13243 


C-1-3 


86 


411 


774 


1678 


3244 








120 


3244 


C-1-4 


80 


399 


775 














31 


800 


D-1-1 


81 


408 


772 


1546 


3314 


6616 


13141 


485 


13274 


D-1-2 


66 


350 


727 


1628 


3285 


6559 





468 


12806 


D-1-3 


86 


411 


774 


1678 


3244 


6524 





250 


6805 


D-1-4 


80 


399 


775 


1570 











63 


1639 


E-1-1 


84 


418 


775 


1579 


3320 


6638 


13163 


486 


13322 


E-l-2 


84 


418 


775 


1625 


3328 


6628 





470 


12904 


E-1-3 


86 


411 


774 


1678 


3244 


6524 





250 


6805 


E-1-4 


80 


399 


775 


1570 











63 


1639 


F-1-1 


84 


418 


778 


1553 


3288 


6479 





395 


10787 


F-1-2 


84 


418 


778 


1553 


3288 


6606 


13131 


485 


13250 


F-1-3 


86 


411 


774 


1678 


3244 


6524 





250 


6805 


F-1-1) 


80 


399 


775 


1570 











63 


1639 


C-1-1 


84 


418 


778 


1531 


3300 


6598 


13122 


486 


13267 


G-1-2 


84 


418 


778 


1535 


3301 


6602 


13126 


485 


13245 


G-1-3 


86 


411 


774 


1678 


3244 


6524 





250 


6805 


G-l-4 


80 


399 


775 


1570 











63 


1639 


H-1-1 


81 


413 


774 


1525 


3221 


6528 


13054 


486 


1 3198 


H-1-2 


85 


409 


758 


1520 


3219 


6525 


13043 


483 


13117 


H-1-3 


86 


411 


774 


1678 


3244 


6524 





250 


6805 


H-1-4 


80 


399 


775 


1570 











63 


1639 


A-2-1 


77 


414 


841 


1693 


3336 


6499 





424 


11313 


A-2-2 


77 


414 


801 


1656 


3299 


6451 





423 


11249 


A-2-3 


72 


356 


743 


1598 


3241 


6393 





247 


6581 


A-2-4 


81 


413 


799 














59 


1627 


B-2-1 


77 


414 


841 


1700 


3322 


6423 





415 


11101 


B-2-2 


77 


414 


801 


1658 


3284 


6379 





415 


11057 


B-2-3 


72 


356 


783 


1642 


3264 


6348 





250 


6654 


B-2-4 


81 


413 


799 














59 


1635 


C-2-1 


77 


414 


840 


1702 


3379 


6512 





419 


1 1214 


C-2-2 


77 


414 


840 


1705 


3292 


6441 





396 


10527 


C-2-3 


72 


356 


783 


1644 


3318 


6419 





247 


6650 


C-2-4 


86 


412 


831 














57 


1620 


D-2-1 


77 


414 


841 


1676 


3318 


6455 





420 


11191 


D-2-2 


77 


414 


801 


1638 


3281 


6416 





419 


11127 


D-2-3 


72 


356 


743 


1581 


3223 


6358 





247 


6584 


D-2-4 


81 


413 


799 














59 


1610 


E-2-1 


77 


414 


841 


1712 


3322 


6489 





426 


11369 


E-2-2 


77 


414 


801 


1677 


3283 


6454 





426 


11322 


E-2-3 


72 


356 


743 


1619 


3225 








120 


3225 


E-2-4 


81 


413 


799 














59 


1655 


F-2-1 


77 


414 


841 


1677 


3317 








234 


6268 


F-2-2 


77 


414 


801 


1639 


3277 


6381 





405 


10689 


F-2-3 


72 


356 


743 


1581 


3219 








233 


6146 


F-2-4 


81 


413 


799 














59 


1612 


G-2-1 


77 


414 


850 


1728 


3407 


6563 





423 


11364 


G-2-2 


77 


414 


829 


1701 


3374 


6534 





422 


1 1308 


G-2-3 


72 


356 


772 


1643 


3316 


6476 





248 


6687 


G-2-4 


81 


413 


849 














59 


1698 


H-2-1 


77 


414 


841 


1677 


3303 


6401 





414 


11055 


H-2-2 


77 


414 


801 


1637 


3265 


6386 





417 


1 1088 


H-2-3 


72 


356 


743 


1580 


3209 








234 


6160 


H-2-4 


81 


413 


799 














59 


1612 


A-3-1 


65 


435 


867 


1692 


3537 


6937 





479 


13680 


A-3-2 


65 


435 


867 


1692 


3537 


6937 





479 


13680 


B-3-1 


78 


362 


708 


1573 


3147 


6724 





468 


13137 


B-3-2 


72 


356 


702 


1566 


3140 


6718 





468 


13131 


C-3-1 


78 


362 


708 


1578 


3184 


6733 





478 


13419 


C-3-2 


72 


356 


702 


1555 


3129 


6707 





351 


9564 


D-3-1 


65 


435 


873 


1698 


3544 


6943 





479 


13687 


D-3-2 


65 


435 


873 


1698 












E-3-1 


83 


426 


836 


1707 


3583 


6668 





422 


10782 


E-3-2 


83 


426 


836 


1707 


3583 


6668 





422 


10782 


F-3-1 


65 


435 


873 


1698 


3544 


6943 





479 


13687 


F-3-2 


65 


435 


873 


1698 


3544 


6943 





479 


13687 


G-3-1 


83 


426 


836 


1707 


3583 


6668 





430 


12995 


G-3-2 


83 


426 


836 


1707 


3583 


6668 





430 


12995 


H-3-1 


78 


362 


708 


1573 


3147 


6724 





468 


13137 


H-3-2 


72 


356 


702 


1566 


3140 


6718 





468 


13131 


A-4-1 


108 


388 


864 


1702 


3665 


7049 





361 


11103 


A-4-2 


108 


388 


864 


1702 


3665 


7049 





361 


11103 


B-4-1 


66 


360 


823 


1694 


3510 


7458 





460 


14172 


B-4-2 


66 


360 


823 


1694 


3510 


7458 





460 


14172 


C-4-1 


64 


371 


842 


1802 


3558 


7375 





425 


13146 


C-4-2 


64 


371 


842 


1802 


3558 


7375 





425 


13146 


D-4-1 


66 


360 


823 


1694 


3510 


7458 





460 


14172 


D-4-2 


66 


360 


823 


1694 


3510 


7458 





460 


14172 


E-4-1 


96 


534 


958 


1875 


3817 


7193 





361 


11246 


E-4-2 


96 


534 


958 


1875 


3817 


7193 





361 


1 1246 


F-4-1 


96 


534 


958 


1875 


3817 


7193 





361 


11246 


F-4-2 


96 


534 


958 


1875 


3817 I 


7193 1 





361 


1 1246 


G-4-1 


108 


388 


864 


1702 


3665 


7049 





361 


1 1103 


G-4-2 


108 


388 


864 


1702 


3665 1 


7049 





361 ! 


1 1103 


H-4-1 


64 


371 


842 


1802 


3558 1 


7375 





425 


13146 


H-4-2 


64 


371 


842 


1802 


3558 


7375 1 


I 


425 1 


13146 



Days with a Minimum Solar Radiation Level of 17,000 kj/sq m 



131 



Table A. 1 . Exposure Summation for all Collectors (Btu/sq ft) 

Exposure Days* 
3 15 30 60 120 240 



Co I I ector 

ID 



480 



Tota I 
Days 



Tota I 
Exposure 



A-1-1 


7288 


36495 


1 68259 


134509 


I 289982 


I 580668 


1 1155180 


I 485 


1 1166860 


A-1-2 


7288 


36495 


67886 


134369 


290000 


I 580488 


1 1155107 


I 485 


1 1166787 


A-l-3 


7612 


36264 


68183 


147772 


285675 


I 574503 


1 


I 250 


1 599264 


A-1-4 


7122 


35169 


68319 


138256 





I 


1 


I 63 


1 144389 


B-1-1 


7159 


36366 


68012 


138754 


291059 


I 583514 


I 1158071 


488 


1 1176745 


B-1-2 


6893 


35860 


66520 


137654 


290871 


583322 


t 1157906 


I 485 


1 1169586 


B-1-3 


7612 


36264 


68183 


147772 


285675 


574503 


I 


250 


1 599264 


B-1-4 


7122 


35169 


68319 


138256 








I 


I 63 


1 144389 


C-1-1 


7159 


36366 


67757 


134369 


287362 


580429 


I 1157029 


487 


1 1173271 


C-1-2 


6893 


35834 


66627 


133107 


288562 


582411 


1156098 


484 


I 1166112 


C-l-3 


7612 


36264 


68183 


147772 


285675 








120 


1 285675 


C-1-4 


7122 


35169 


68319 














31 


1 70467 


D-1-1 


7159 


35989 


68015 


136159 


291823 


I 582609 


1157193 


1 485 


1 1168873 


D-1-2 


5823 


30874 


64081 


143390 


289324 


I 577597 





1 468 


1 1127655 


D-1-3 


7612 


36264 


68183 


147772 


285675 


574503 





250 


1 599264 


D-.1-4 


7122 


35169 


68319 


138256 











63 


1 144389 


E-1-1 


744 3 


36886 


68277 


139086 


292403 


584559 


1159098 


486 


1 1173081 


E-1-2 


7443 


36886 


68277 


143126 


293104 


583677 





470 


1 1136334 


E-1-3 


7612 


36264 


68183 


147772 


285675 


574503 





250 


I 599264 


E-1-4 


7122 


35169 


68319 


138256 











63 


144389 


F-1-1 


7443 


36886 


68532 


136761 


289587 


570586 





395 


949858 


F-1-2 


7443 


36886 


68532 


136761 


289587 


581743 


1156282 


485 


1166745 


E-1-3 


7612 


36264 


68183 


147772 


285675 


574503 





250 


599264 


F-1-4 


7122 


35169 


68319 


138256 











63 


144389 


G-1-1 


7443 


36886 


68532 


134815 


290602 


581017 


1 155501 


486 


1168223 


G-1-2 


7443 


36886 


68532 


135229 


290702 


581388 


1155871 


485 


11663 34 


G-1-3 


7612 


36264 


68183 


147772 


285675 


574503 





250 


599264 


G-1-4 


7122 


35169 


68319 


138256 











63 


144389 


11-1-1 


7159 


36381 


68241 


134322 


283658 


574898 


1 149503 


486 


1 162225 


H-1-2 


7498 


36089 


66749 


133857 


283484 


574573 


1 148572 


483 


1155066 


H-1-3 


7612 


36264 


68183 


147772 


285675 


574503 





250 


599264 


H-1-4 


7122 


35169 


68319 


138256 











63 


144389 


A-2-1 


6857 


36487 


74097 


149119 


293778 


572275 





424 


996209 


A-2-2 


6861 


36491 


70535 


145847 


290526 


568098 





423 


990531 


A-2-3 


6371 


31402 


65446 


140 758 


285437 


56 3009 





247 


579571 


A-2-4 


7174 


36388 


70432 














59 


14 33 38 


B-2-1 


6857 


36487 


74097 


149758 


292546 


565638 





4 15 


977500 


B-2-2 


6861 


36491 


70535 


146077 


289204 


561784 





415 


973646 


B-2-3 


6371 


31402 


69012 


144673 


287461 


559044 





250 


585994 


B-2-4 


7174 


36388 


704 32 














59 


143977 


C-2-1 


6857 


36487 


740 34 


149898 


297570 


573490 





419 


987451 


C-2-2 


6861 


36491 


74038 


150154 


289945 


567197 





396 


926953 


C-2-3 


6371 


31402 


68949 


144813 


292209 


565304 





247 


585647 


C-2-4 


7612 


36299 


73223 














57 


142663 


D-2-1 


6857 


36487 


74097 


147587 


292246 


568447 





420 


985492 


D-2-2 


6861 


36491 


70535 


144315 


288960 


564973 





419 


979814 


D-2-3 


6371 


31402 


65446 


139226 


283871 


559884 





247 


579808 


D-2-4 


7174 


36388 


70432 














59 


141806 


E-2-1 


6857 


36487 


74097 


150763 


292590 


571414 





426 


1001165 


E-2-2 


6861 


36491 


70535 


147689 


289120 


568372 





426 


996961 


E-2-3 


6371 


31402 


65446 


142600 


284031 








120 


284031 


E-2-4 


7174 


36388 


70432 














59 


145792 


F-2-1 


6857 


36487 


74097 


147731 


292123 








234 


552013 


F-2-2 


6861 


36491 


70535 


144332 


288574 


561924 





405 


941252 


F-2-3 


6371 


31402 


65446 


139243 


283485 








233 


541246 


F-2-4 


7174 


36388 


70432 














59 


141950 


G-2-1 


6857 


36487 


74906 


152180 


300018 


577986 





423 


1000657 


G-2-2 


6861 


36491 


73076 


149817 


297130 


575364 





422 


995793 


G-2-3 


6371 


31402 


67987 


144728 


292041 


570275 





248 


588872 


G-2-4 


7174 


36388 


74807 














59 


149583 


H-2-1 


6857 


36487 


74097 


147731 


290910 


563694 





414 


973528 


H-2-2 


6861 


36491 


70535 


144230 


287580 


562396 





417 


976371 


H-2-3 


6371 


31402 


65446 


139141 


282569 








234 


542459 


H-2-4 


7174 


36388 


70432 








O 





59 


141950 


A-3-1 


5794 


38379 


76371 


148998 


311492 


610851 





479 


1204670 


A-3-2 


5794 


38379 


76371 


148998 


31 1492 


610851 





479 


1204670 


B-3-1 


6954 


31958 


62374 


138523 


277122 


592133 





468 


1156794 


B-3-2 


6411 


31415 


61831 


137980 


276579 


591590 





468 


1 156251 


C-3-1 


6954 


31958 


62374 


138980 


280432 


592933 





478 


1181613 


C-3-2 


6411 


31415 


61831 


137007 


275606 


590617 





351 


842229 


D-3-1 


5794 


38379 


76951 


149578 


3 12072 


61 1431 





479 


1205250 


0-3-2 


5794 


38379 


76951 


149578 


312072 


6114 31 





479 


1205250 


E-3-1 


7392 


37517 


73633 


150336 


315580 


587199 





422 


949447 


E-3-2 


7392 


37517 


73633 


150336 


315580 


587199 





422 


949447 


F-3-1 


5794 


38379 


76951 


149578 


312072 


61 1431 





479 


1205250 


F-3-2 


5794 


38379 


76951 


149578 


312072 


6114 31 





479 


1205250 


G-3-1 


7392 


37517 


73633 


150336 


315580 


587199 





430 


1 144325 


C-3-2 


7392 


37517 


73633 


150336 


315580 


587199 





430 


1144325 


H-3-1 


6954 


31958 


62374 


138523 


277122 


592133 





468 


1156794 


H-3-2 


6411 


31415 


61831 


137980 


276579 


591590 





468 


1156251 


A-4-1 


9524 


34227 


76130 


149950 


322755 


620736 





361 


977752 


A-4-2 


9524 


34227 


76130 


149950 


322755 


620736 





361 


977752 


B-4-1 


5820 


31735 


72488 


149222 


309135 


656724 





460 


1247989 


B-4-2 


5820 


31735 


72488 


149222 


309135 


656724 





460 


1247989 


C-4-1 


5713 


32752 


74214 


158700 


313355 


649423 





425 


1157650 


C-4-2 


5713 


32752 


74214 


158700 


313355 


649423 





425 


1 157650 


D-4-1 


5820 


31735 


72488 


149222 


309135 


656724 





460 


1247989 


D-4-2 


5820 


31735 


72488 


149222 


309135 


656724 





460 


1247989 


E-4-1 


8493 


47059 


84388 


165130 


336187 


633416 





361 


990331 


E-4-2 


8493 


47059 


84388 


165130 


336187 


63 34T6 





361 


990331 


F-4-1 


8493 


47059 


84388 


165130 


336187 


633416 


I 


361 


9903 31 


F-4-2 


8493 


47059 


84388 


165130 


336187 


633416 


j 


361 


9903 31 


G-4-1 


9524 


34227 


76130 


149950 


322755 


620736 


I 


361 1 


977752 


G-4-2 


9524 


34227 


76130 


149950 


322755 


620736 


I 


361 1 


977752 


H-4-1 


5713 


32752 


74214 


158700 


313355 


649423 


I 


425 1 


1157650 


H-4-2 


5713 


32752 


74214 1 


158 700 I 


313355 I 


649423 1 


I 


425 1 


1 157650 



* Days with a Minimum Solar Radiation Level of 1,500 Btu/sq ft 



132 



Table A. 2. Exposure Summation for Material Samples at Phoenix 

A. Cover Samples Miniboxes 

Exposure Schedule: Started 5- 8-79 Ended 11-18-80 

( 2-12-80 through 3- 3-80 Excluded) 



Ca I enda r 


Exposure 


Date 


Tota I So I a r 


Rad i a t ion 


Days 


Days* 




Btu/sq ft 


MJ/sq m 


85 


80 


7-30-79 


194587 


2209 


170 


160 


10-23-79 


371472 


4218 


283 


240 


3- 5-80 


565881 


6426 


372 


320 


6- 2-80 


759847 


8629 


454 


400 


8-23-80 


953122 


10823 


540 


479 


1 1-17-80 


1127756 


12807 



B. Absorber Sample Coupons AC12 - AH12, AJ12, AL12, & AP12 
Exposure Schedule: Started 6- 4-79 Ended 6-14-81 

( 3-24-80 through 8-28-80 Excluded) 



Ca I enda r 


Exposure 


Date 


Tota I So I a r 


Rad i at i on 


Days 


Days 




Btu/sq ft 


MJ/sq m 


82 


80 


8-24-79 


188493 


2140 


170 


160 


11-20-79 


367469 


4173 


292 


240 


3-21-80 


565438 


6421 


381 


320 


1 1-23-80 


743729 


8446 


487 


400 


3- 9-81 


934281 


10609 


576 


480 


6- 6-81 


1133008 


12866 


584 


488 


6-14-81 


1153849 


13103 



C. Absorber Sample Coupons AI11, AA12, AM12, AN12, AN13, & AA15 - AAP15 
Exposure Schedule: Started 6- 4-79 Ended 6-14-81 

( 3-24-80 through 1- 3-81 Excluded) 



Ca I enda r 


Exposure 


Date 


Tota I So I a r 


Rad ia t i on 


Days 


Days 




Bt 


u/sq ft 


MJ/sq m 


82 


80 


8-24-79 




188493 


2140 


170 


160 


11-20-79 




367469 


4173 


292 


240 


3-21-80 




565438 


6421 


397 


320 


4-16-81 




764755 


8684 


456 


376 


6-14-81 




902626 


10250 



* Days with a Minimum Solar Radiation Level of 17,000 kJ/sq m 



133 



Table A. 3. Exposure Summation for Material Samples at Cape Canaveral 

A. Cover Sample Miniboxes 

Exposure Schedule: Started 4- 6-79 Ended 2-10-81 



Ca I enda r 


Exposure 


Date 


Tota I So I a r 


Rad i a t i 


Days 


Days* 




Btu/sq ft 


MJ/sq m 


97 


80 


7-11-79 


188013 


2135 


199 


160 


10-21-79 


373420 


4240 


328 


240 


2-27-80 


571479 


6489 


435 


320 


6-13-80 


770512 


8750 


528 


400 


9-14-80 


951532 


10805 


645 


480 


1- 9-81 


1 141726 


12965 


677 


500 


2-10-81 


1194989 


13570 



B. Absorber Sample Coupons 

Exposure Schedule: Started 5- 2-79 Ended 10-24-81 

( 4-14-8O through 12-20-80 Excluded) 



Ca I enda r 


Exposure 


Date 


Tota 1 So 1 a r 


Rad i at i 


Days 


Days 




Btu/sq ft 


MJ/sq m 


93 


80 


8- 2-79 


18281 1 


2076 


203 


160 


11-20-79 


372875 


4234 


330 


240 


3-26-80 


571779 


6493 


455 


320 


4- 6-81 


781735 


8877 


541 


400 


7- 1-81 


967635 


10988 


638 


480 


10- 6-81 


1149935 


13059 


656 


494 


10-24-81 


1182464 


13428 



* Days with a Minimum Solar Radiation Level of 17,000 kJ/sq m 



134 



Table A. 4. Exposure Summation for Material Samples at Palo Alto 



A. Cover Sample Mini boxes 
Exposure Schedule: Started 4- 



6-79 Ended 7-28-81 



Ca I enda r 


Exposure 


Date 


Tota 1 So 1 a r 


Rad i a t i on 


Days 


Days* 




Btu/sq ft 


MJ/sq m 


88 


80 


7- 2-79 


200822 


2280 


172 


160 


9-24-79 


390452 


4434 


342 


240 


3-12-80 


612243 


6952 


437 


320 


6-15-80 


803841 


9128 


519 


400 


9- 5-80 


989828 


1 1240 


666 


480 


1-30-81 


1196403 


13586 


782 


560 


5-26-81 


1399242 


15890 


845 


623 


7-28-81 


1549724 


17599 



B. Absorber Sample Coupons 

Exposure Schedule: Started 5- 2-79 Ended 7-28-81 

( 5-22-80 through 5-29-80 Excluded) 



Ca I enda r 
Days 


Exposure 
Days 


Date 


Tota I So la r 
Btu/sq ft 


Rad i at i on 
MJ/sq m 


83 


80 


7-23-79 


199394 


2264 


178 


160 


10-26-79 


391549 


4446 


341 


240 


4- 6-80 


609729 


6924 


433 


320 


7-15-80 


803517 


9124 


517 


400 


10- 7-80 


981376 


11144 


690 


480 


3-29-81 


1207844 


13716 


776 


560 


6-23-81 


13981 19 


15877 


811 


595 


7-28-81 


1481065 


16819 



* Days with a Minimum Solar Radiation Level of 17,00 kj/sq m 



135 



Table A. 5 Exposure Summation for Material Samples at Ga i thersburg 

A. Cover Sample Mini boxes 

Exposure Schedule: Started 5- 5-79 Ended 6- 8-81 



Ca I enda r 


Exposure 


Date 


Tota I So I a r 


Rad i at i 


Days 


Days'* 




Btu/sq ft 


MJ/sq m 


130 


80 


9-11-79 


213024 


2419 


320 


160 


3-19-80 


443133 


5032 


443 


240 


7-20-80 


645341 


7328 


575 


320 


11-29-80 


848138 


9631 


766 


397 


6- 8-81 


1083173 


12300 



B. Absorber Sample Coupons 

Exposure Schedule: Started 5- 5-79 Ended 6- 8-81 

( 8-15-80 through 12- 5-80 Excluded) 

Total Solar Radiation 
Btu/sq ft MJ/sq m 

213024 2419 

443133 5032 

645341 7328 

875453 9941 

918297 10428 

* Days with a Minimum Solar Radiation Level of 17,000 kj/sq m 



Ca I enda r 


Exposure 


Date 


Days 


Days 




130 


80 


9-11-79 


320 


160 


3-19-80 


443 


240 


7-20-80 


622 


320 


5- 8-81 


653 


336 


6- 8-81 



136 



NBS-114A (REV. 2-80) 



U.S. DEPT. OF COMM. 

BIBLIOGRAPHIC DATA 

SHEET (See instructions) 



1. PUBLICATION OR 
REPORT NO. 

NBS/TN-1196 



2. Performing Organ. Report No 



3. Publ ication Date 



September 1984 



4. TITLE AND SUBTITLE 

NBS Solar Collector Durability /Reliability Test Program: Final Report 



5. AUTHOR(S) 

David Waksman, William C. Thomas, Elmer R. Streed 



6. PERFORMING ORGANIZATION (If joint or other than NBS, see instructions) 

NATIONAL BUREAU OF STANDARDS 
DEPARTMENT OF COMMERCE 
GAITHERSBURG, MD 20899 



7. Contract/Grant No. 



8. Type of Report & Period Covered 

Final 



9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State, ZIP) 

Office of Solar Heat Technologies 
U.S. Department of Energy 
Washington, DC 20585 



10. SUPPLEMENTARY NOTES 



J Document describes a computer program; SF-185, FlPS Software Summary, is attached. 

11. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant 
bibliography or literature survey, mention it here) 

Efforts in the development of reliability/durability tests for solar collectors and 
their materials have been hampered by the lack of real time and accelerated degrada- 
tion data that can be correlated with in use conditions. The focus of this report 
is on research undertaken at the National Bureau of Standards (NBS) to help generate 
the data required to develop methods for predicting the long term durability and 
reliability of flat-plate solar collectors and their materials. 

In this research, eight different types of flat-plate solar collectors were exposed 
outdoors at four sites located in different climatic regions. Small scale cover and 
absorber materials coupon specimens consisting of samples taken from a collector of 
each of the eight types used and a number of additional materials were exposed con- 
currently with the full-size collectors. Periodic measurements were made of collec- 
tor and materials performance as a function of outdoor exposure time. Indoor labora- 
tory aging tests were conducted concurrently on specimens of the same materials to 
provide a basis for comparison with the outdoor exposure tests. 

This report presents the results obtained in this test program. Recommendations are 
made regarding the use and limitations of performance measurements and environmental 
exposure tests for assessing the durability of solar collectors and absorber and 
cover materials. 



12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons) 

absorber materials; accelerated aging; cover materials; durability; environmental 
exposure; solar collectors; solar materials, stagnation testing, thermal performance, 



13. AVAILABILITY 

[Xj Unlimited 

| | For Official Distribution. Do Not Release to NTIS 

[Xl Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 
20402. 

~ ] Order From National Technical Information Service (NTIS), Springfield, VA. 22161 



14. NO. OF 

PRINTED PAGES 

148 



15. Price 



USCOMM-DC 6043-P80 



*U.S. GOVERNMENT PRINTING OFFICE : 1984 0-420-997/10038 



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